Add a flushBuffer Connection API

[ColinSullivan1]

Some clients have a buffer flush API that flushes the local outbound buffer without sending a ping/pong. This is useful in reducing latency of publish calls while avoiding the full flush with a ping/pong.

e.g.

nats.net: flushBuffer()
nats.c: natsConn_bufferFlush()

We'll want to add an equivalent API to this java client.

CC /@scottf

[ColinSullivan1]

Resolved by #380