bdr: Fix typo causing the send/recv not to be used even though version's match

Found in respond to tests by Christian.
bdr: Fix typo causing the wrong tuple format to be sent
Rewrote lookup code for bdr.bdr_queued_commands

Now we're looking up the QueuedDDLCommandsRelid of
bdr.bdr_queued_commands in bdr_maintain_schema() using
get_namespace_oid() as well as get_relname_relid().