txn_limbo: persist the latest effective promote in snapshot

Previously PROMOTE entries, just like CONFIRM and ROLLBACK were only
stored in WALs. This is because snapshots consist solely of confirmed
transactions, so there's nothing to CONFIRM or ROLLBACK.

PROMOTE has gained additional meaning recently: it pins limbo ownership
to a specific instance, rendering everyone else read-only. So now
PROMOTE information must be stored in snapshots as well.

Save the latest limbo state (owner id and latest confirmed lsn) to the
snapshot as a PROMOTE request.

Prerequisite #6034