bdr: Improve sequencer locking and pgstat reporting.

bdr: Improve sequencer locking and pgstat reporting.

Up to now the sequencer locked its relations exclusively to prevent an
interlock with apply. That was problematic, because it would often
prevent auto vacuum/analyze from running, or interrupt it.

Instead of locking the underlying relations lock the bdr seqam object
and add code to apply to also acquire that lock. That allow autovacuum
to run undisturbed.

Testing brought an additional problem to light, namely that pgstat
records weren't sent in the sequencer until shutdown, which means that
in many situations autovacuum won't be trigger in time. This is a more
widespread problem than just the sequencer, but here only the sequencer
is fixed.