Don't unserialize integer reply from Redis incrBy

It does not work with unserialize because RedisBagOStuff->unserialize
expects either strings of digits, or serialized strings.  A plain integer
is neither.

incr docs were incorrect, so fix and expand by copying from parent class.

Change-Id: Ifc9c7a36f221b251ceea0d9a79ae6c3ce5753dd4