Adding long heartbeat timeouts on connections

Newer redis versions appear to be sending PINGs from time to time to
make sure the communication is open. On older copies, though, there is
no heartbeat and thus no way to know if the communication is just silent
or down (without a reset packet having been sent to notify the reader).

This patch adds a heartbeat with a long delay (15 minutes +) that will
shut down nsync (and let a supervisor restart it) if the server doesn't
answer to a PING command (with "+PONG\r\n") in the next few minutes.