JCBC-250: Fail Fast when Observe with Memcached Bucket is used.

This changeset makes sure that an obviously illegal operation
(using persist/repliace with memcached type buckets) fails
fast and with a expressive IllegalArgumentException and not
further down the stack with a ClassCastException as reported
in the JCBC ticket.

Change-Id: Icb6acc019df737a7f2b10bc83f9a303594fd8369
Reviewed-on: http://review.couchbase.org/26541
Reviewed-by: Deepti Dawar <deepti.dawar@globallogic.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>