Bug fix: slaves being pinged every second

Bug fix: slaves being pinged every second

REDIS_REPL_PING_SLAVE_PERIOD controls how often the master should
transmit a heartbeat (PING) to its slaves.  This period, which defaults
to 10, is measured in seconds.

Redis 2.4 masters used to ping their slaves every ten seconds, just like
it says on the tin.

The Redis 2.6 masters I have been experimenting with, on the other hand,
ping their slaves *every second*.  (master_last_io_seconds_ago never
approaches 10.)  I think the ping period was inadvertently slashed to
one-tenth of its nominal value around the time REDIS_HZ was introduced.
This commit reintroduces correct ping schedule behaviour.