CCBC-152: Allow to check for existence of a key

This adds a new version to the observe command structure which allows to
specify that the request be made to the master only.

Change-Id: I96debd0a14586c0a0578b968759d92b3420a0ac7
Reviewed-on: http://review.couchbase.org/33392
Reviewed-by: Brett Lawson <brett19@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>