Implement drain functionality, initial untested commit, DRAFT ONLY

[jsudano]

Details at nats-io#1325

[Jarema]

Hey.
Just to give you early feedback - I think it looks well.
Obviously requires quite a lot of testing.

[jsudano]

Awesome thank you, I'll get to writing tests then!

Is there a "development" doc for the repo? I've gathered that I need to manually run a NATS server locally before running cargo test, but I see test failures related to "no such file or directory" (even on master) when trying to run tests against that local NATS server. I'm assuming there's some setup step I'm missing.

[Jarema]

All you need to do is having nats-server available in you PATH.
The test suite will drive starting, cleaning up and shutting down the server.