Test Realtime Replication Heartbeat feature.

This PR adds a riak_test for the realtime heartbeat, which works like this...

If both sides support heartbeat message, it is sent from the RT source
every {riak_repl, rt_heartbeat_interval} which default to 15s.  If
a response is not received in {riak_repl, rt_heartbeat_timeout}, also
default to 15s then the source connection exits and will be re-established
by the supervisor.

The test reduces the heartbeat interval to 2 seconds so that test will converge more quickly. This did uncover a typo in the RT source code, which is addressed by the following PR:

https://github.com/basho/riak_repl/pull/312