Unsubscribe fix

[jerel]

The current implementation of juniper_warp will kill the websocket at the first stop call or if a deserialization error is encountered. This means that any running operation (such as a subscription) that is stopped will require the client to reconnect and set up its subscriptions again.

This fix does two things:

1. ACKs a connection_init command
2. Only closes the websocket if the client sends a connection_terminate command

Both are modeled after the Apollo implementation https://github.com/apollographql/subscriptions-transport-ws/blob/master/PROTOCOL.md

[LegNeato]

@jerel Sorry for this taking so long! Would love a test to make sure we don't regress. Also, will this be obsoleted by #721?

[jerel]

@LegNeato yes, this PR is unnecessary due to #721. I'll close for now and if the new implementation from 721 still works this way when I get around to using it I'll re-open an issue or PR based on that.