session: internal request execution API error types refactor

[muzarski]

Ref: #519
Depends on: #1156

execute_request_once closure

To narrow the error return types of some important API functions (e.g. RS::decide_should_retry and LBP::on_query_failure), the error type of execute_request_once (previously known as do_query) needs to be adjusted as well. A couple of commits at the beginning of this PR do just that. What we achieve, is that we narrow the return error type of the closure from QueryError to UserRequestError.

LBP and RetrySession

Both of these traits accept some error and do something based on the error. Before this PR, they would accept a QueryError, resulting in matches against variants that could never even be constructed before the call. In this PR, I pub-ified the UserRequestError, and made LBP::on_query_failure and RetrySession::decide_should_retry accept it instead of QueryError.

Pre-review checklist

• I have split my patch into logically separate commits.
• All commit messages clearly explain what they change and why.
• I added relevant tests for new features and bug fixes.
• All commits compile, pass static checks and pass test.
• PR description sums up the changes and reasons why they should be introduced.
• I have provided docstrings for the public items that I want to introduce.
• [ ] I have adjusted the documentation in ./docs/source/.
• I added appropriate Fixes: annotations to PR description.

[github-actions]

cargo semver-checks detected some API incompatibilities in this PR.
Checked commit: 22c90e4

See the following report for details:

cargo semver-checks output

./scripts/semver-checks.sh --baseline-rev 847e44d61e65eccb6205bd586ccaa808e57a6c19
+ cargo semver-checks -p scylla -p scylla-cql --baseline-rev 847e44d61e65eccb6205bd586ccaa808e57a6c19
     Cloning 847e44d61e65eccb6205bd586ccaa808e57a6c19
    Building scylla v0.15.0 (current)
       Built [  23.047s] (current)
     Parsing scylla v0.15.0 (current)
      Parsed [   0.054s] (current)
    Building scylla v0.15.0 (baseline)
       Built [  21.862s] (baseline)
     Parsing scylla v0.15.0 (baseline)
      Parsed [   0.052s] (baseline)
    Checking scylla v0.15.0 -> v0.15.0 (no change)
     Checked [   0.122s] 107 checks: 105 pass, 2 fail, 0 warn, 0 skip

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/struct_missing.ron

Failed in:
  struct scylla::transport::retry_policy::QueryInfo, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-847e44d61e65eccb6205bd586ccaa808e57a6c19/389a3366a0a28da0298589ce4c12e851cb8631c2/scylla/src/transport/retry_policy.rs:9
  struct scylla::retry_policy::QueryInfo, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-847e44d61e65eccb6205bd586ccaa808e57a6c19/389a3366a0a28da0298589ce4c12e851cb8631c2/scylla/src/transport/retry_policy.rs:9

--- failure trait_method_missing: pub trait method removed or renamed ---

Description:
A trait method is no longer callable, and may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#major-any-change-to-trait-item-signatures
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/trait_method_missing.ron

Failed in:
  method on_query_success of trait LoadBalancingPolicy, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-847e44d61e65eccb6205bd586ccaa808e57a6c19/389a3366a0a28da0298589ce4c12e851cb8631c2/scylla/src/transport/load_balancing/mod.rs:83
  method on_query_failure of trait LoadBalancingPolicy, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-847e44d61e65eccb6205bd586ccaa808e57a6c19/389a3366a0a28da0298589ce4c12e851cb8631c2/scylla/src/transport/load_balancing/mod.rs:86
  method on_query_success of trait LoadBalancingPolicy, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-847e44d61e65eccb6205bd586ccaa808e57a6c19/389a3366a0a28da0298589ce4c12e851cb8631c2/scylla/src/transport/load_balancing/mod.rs:83
  method on_query_failure of trait LoadBalancingPolicy, previously in file /home/runner/work/scylla-rust-driver/scylla-rust-driver/target/semver-checks/git-847e44d61e65eccb6205bd586ccaa808e57a6c19/389a3366a0a28da0298589ce4c12e851cb8631c2/scylla/src/transport/load_balancing/mod.rs:86

     Summary semver requires new major version: 2 major and 0 minor checks failed
    Finished [  46.553s] scylla
    Building scylla-cql v0.4.0 (current)
       Built [  10.457s] (current)
     Parsing scylla-cql v0.4.0 (current)
      Parsed [   0.036s] (current)
    Building scylla-cql v0.4.0 (baseline)
       Built [  10.844s] (baseline)
     Parsing scylla-cql v0.4.0 (baseline)
      Parsed [   0.036s] (baseline)
    Checking scylla-cql v0.4.0 -> v0.4.0 (no change)
     Checked [   0.111s] 107 checks: 107 pass, 0 skip
     Summary no semver update required
    Finished [  22.311s] scylla-cql
make: *** [Makefile:61: semver-rev] Error 1

[muzarski]

v1.1: Rebased on latest #1156 and addressed @Lorak-mmk comments. Most important change is that we replaced From<UserRequestError> for QueryError with explicit conversion method UserRequestError::into_query_error().

[Lorak-mmk]

Looks good.

A suggestion for the future (because this PR is probably not a good place for this): could you re-order variants in UserRequestError so that they match the order in QueryError? Changing the QueryError instead would be fine too - I just think that the variants present in both should appear in the same order in both.