JCBC-319: check config for memcache buckets.

This changeset makes sure that when using ketama with
memcache buckets, failing operations also trigger
reconfiguration eventually. Note that this algorithm is
different than for the CouchbaseConnection, because ketama will
always find a next node. Therefore, we trigger reconfigure but let
the doc pass through to a separate node.

Change-Id: I83ce342b76358a374469ca27e3807d8c3d8381af
Reviewed-on: http://review.couchbase.org/27065
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>