observe_t: check for lcb >= 2.3.0 for observe-master-only

This feature is not available on older lcb versions

Change-Id: I06a468f71503ce350edbb94c5d4240f428dd1c3c
Reviewed-on: http://review.couchbase.org/37078
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>