Refactor Endpoint Builder

[ecton]

The Endpoint Builder interface is not really friendly to use. Until the max_idle_timeout setting, I was using the simple new_server/new_client methods, but I can't use those simple methods in conjunction with the new configuration value.

The primary issue I have with the builder type is that it doesn't actually follow either of the common builder patterns -- the modification methods don't return self, preventing chaining the calls. This yielded a pretty ugly net diff:

khonsulabs/bonsaidb@2ee96a2

Ideally, these could be refactored into something like:

Endpoint::builder().server(port, keypair).max_idle_timeout(None).build()?