Cleanup of MemcachedClient.

This changeset adds more @Override annotations and does some minor
cleanup.

Change-Id: I1a9140635781bf622c6aeba4156f5e1cb7f5b468
Reviewed-on: http://review.couchbase.org/31670
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>