txcouchbase: Allow first argument to be connection string

Change-Id: I78d2d31a48e48921ebfa3cf65c2343679b8fd749
Reviewed-on: http://review.couchbase.org/43609
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>