RCBC-131 Couchbase::Cluster shouldn't require persistent connection

Change-Id: I12328da34d92239f9321acdd0d671bdff568f135
Reviewed-on: http://review.couchbase.org/26710
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>