add TLS support to HalfCloser interface

[mikemorris]

This relates to a known issue documented in #6203, and may fix TLS connections to half-close properly.

When I started writing this I had assumed CloseWrite would exist on the returned net.Conn object and wouldn't need to cast in agent/consul/snapshot_endpoint.go, looking back may want to re-implement HalfCloser, just over either connection type now?

https://github.com/hashicorp/consul/blob/02a52ef22881bcf98184a9604294aacfaf846c09/agent/consul/snapshot_endpoint.go#L225-L244

Refs golang/go#8579

[banks]

looking back may want to re-implement HalfCloser, just over either connection type now?

Yeah I think the old model was fine and we should just return the half closer for tls too. This PR is actually not bad and might even be closer to what I'd have written in the first place but I think it would be a smaller change to keep HalfCloser and make it work for TLS instead.

The (tiny, theoretical) benefit of the interface is that if we ever have other conn types (unlikey) we wouldn't have to change this code to make it work...

[mikemorris]

This should be ready for review again - any suggestions on if/how/where I should write a test for this?

[banks]

Nice! Seems like a simpler change.

I can't think of a good way to test this sadly without either changing the code much more to plumb in dummy diallers or some elaborate, flaky integration test and I don't think either is a good use of time.