JCBC-268: Optimize ReplicateTo.ZERO & PersistTo.ZERO

This changeset optimizes the persistence constraint methods
when both are zero, which basically means do not check at
all. Without this change, added latency is involved because
"observe" is alaways called, even when not necessary.

Change-Id: I8dd6b6226dc797ee066b2113ff7b67f48735baf8
Reviewed-on: http://review.couchbase.org/26182
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>