Check EpochRewards::active within the Stake Program

[CriesofCarrots]

Problem

As per SIMD-0118, credits to stake accounts should be allowed during the rewards interval. The current implementation Errors all transactions that load a stake account as writable, overly restrictive.

Summary of Changes

• Check whether EpochRewards::active and return new StakeError for all instructions except GetMinimumDelegation
  Note: this implements the specification in the SIMD, but I think this could be relaxed on a handful of instructions, like Authorize and SetLockup. It's possible that even Initialize, DelegateStake, and Deactivate could be supported during the rewards interval as well, depending on cache state for recalculation (boot from snapshot). At any rate, unless either of you feel strongly about doing that now, I'll make any of those changes in a separate PR.
• (edit) The following moved to separate PR, which I will link.
  Remove existing check_account_access() call in SVM
  I did leave this method in the TransactionProcessingCallback trait, however. It's a generic-enough name that it could be used for something other than rewards-period restriction.

Towards #426

[CriesofCarrots]

Oh also: I know we need some comprehensive tests of this logic in solana_stake_program now. If you would like that added to this PR, as opposed to done separately, let me know.

(edit) Test added

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.9%. Comparing base (01460ef) to head (df3e81a).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #617    +/-   ##
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  Coverage    81.8%    81.9%            
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  Files         851      851            
  Lines      230238   230394   +156     
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+ Hits       188511   188729   +218     
+ Misses      41727    41665    -62     

[jstarry]

Do you mind splitting this into a change that removes the old logic and a change that introduces the new logic? As for tests, I would like to see at least one more that checks that get min delegation still works during epoch rewards

[CriesofCarrots]

Do you mind splitting this into a change that removes the old logic and a change that introduces the new logic? As for tests, I would like to see at least one more that checks that get min delegation still works during epoch rewards

Removed the old-logic-removal commits from this PR.
Added test that shows that GetMinimumDelegation works (and all other ix fail).

[joncinque]

Looks great!

[joncinque]

cc @2501babe on this change, since the bpf version of the stake program will need to fetch the epoch rewards using the new syscall