pgq: Ability to limit the number of events inserted by one TX

pgq: Ability to limit the number of events inserted by one TX

As PgQ can split batches only on transaction boundaries,
it does not tolerate huge number of events inserted by one TX.

This batch adds per-queue field queue_per_tx_limit, which can be
set to reasonable number and PgQ will enforce the limit, by throwing
error if event counts gets larger, thus rollbacking the problematic TX.

Such limit also adds possibility to survive UPDATE/DELETE statements,
where the WHERE clause is missing or buggy.