cherrypick 294bcfc4d207d025f7dc2b5716b787f03f390332  from AntiRezRedis\2.8
conflicts:
pubsub.c
Variadic PING with support for Pub/Sub.

PING can now be called with an additional arugment, behaving exactly
like the ECHO command. PING can now also be called in Pub/Sub mode (with
one more more subscriptions to channels / patterns) in order to trigger
the delivery of an asynchronous pong message with the optional payload.

This fixes issue #420.
PING: backward compatible error for wrong number of args.
Test: Pub/Sub PING.
Removing the F flag since we have not incorporated latency monitoring changes