fix comments forgotten in #285 (zipmap -> ziplist)
Sentinel: advertise the promoted slave address only after successful setup.
Add SO_KEEPALIVE support to anet.c.
Set SO_KEEPALIVE on client sockets if configured to do so.
tcp-keepalive option documented in redis.conf.
Tcp keep-alive: send three probes before detectin an error.

Otherwise we end with less reliable connections because it's too easy
that a single packet gets lost.
TCP keep-alive. Better documentation in redis.conf.