NCBC-238: Add support for master-only, in-memory observe

Use case allows for supporting KeyExists without Get

Change-Id: I85d99f2798bce3e7390a9ffe18c2cb61fb778cb2
Reviewed-on: http://review.couchbase.org/25292
Reviewed-by: Saakshi Manocha <saakshi.manocha@globallogic.com>
Tested-by: Saakshi Manocha <saakshi.manocha@globallogic.com>