Version 18_2

Add XACT_EVENT_PRE_PROC_ARRAY

A transaction's CSN is assigned by ProcArrayEndTransaction(), while an
extension's XACT_EVENT_COMMIT callback runs afterwards.  An extension
that keeps its own commit state therefore has no place to publish it
together with the transaction's own visibility: everything it can hook
runs either before the CSN exists or after it is already visible.

Fire a new event immediately before ProcArrayEndTransaction(), that is,
in the last moment at which the transaction is still in progress for
everybody else.  The commit is durable by then, so a callback must not
fail: the event is meant for publishing commit state, not for work that
can error out.

OrioleDB uses it to mark an oxid as committing before its CSN becomes
visible, closing a window in which a snapshot could be taken above a
transaction's CSN while OrioleDB still reported it in progress.