Client should provide interface for graceful close of eventstore connections

[Konfuzzyus]

Observed Behavior:
The EsConnection interface does not provide a method to intentionally shut down the connection to the event store. The connection can be severed by terminating the actor system, but this will result in a TCP RST on the connection, prompting an error log on the Eventstore side Close reason: [ConnectionReset] Socket receive error

Expected Behavior:
The EsConnection interface provides a method that will gracefully terminate the connection to the event store, finishing all pending transmissions, then shutting down the TCP connection with TCP FIN.

Further reading:
Also see http://stackoverflow.com/questions/13049828/fin-vs-rst-in-tcp-connections, http://doc.akka.io/docs/akka/2.4/scala/io-tcp.html#Closing_connections