Refactor player connection

Those commits decouple the logic of reading messages from the
service/handler and delegates the responsibility for the acceptor itself
by implementing a GetNextMessage method. TCP Acceptor now parses whole
messages one by one from the stream, this way we ensure not to read
partial messages anymore.

Adding etcd auth (topfreegames#118)

Fix pitaya client interface (topfreegames#117)

Allow client to customize handshake data (topfreegames#116)

Logging errors writing heartbeat

log before every heartbeat

improve agent logs

Change log level of GetSessionFromCtx

Trying again to fix dependencies

Updating go.mod