JCBC-349: Make sure flush() works on memcached buckets.

This changeset enables the pending flush() tests and makes sure
that they also work on memcached type buckets.

Change-Id: I5b1916c7e3058a819c0787c60bad3cc8b3df4c21
Reviewed-on: http://review.couchbase.org/28599
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>