Introduce a new routine, set_next_tx_row(), which checks tx boundary
violation and appends the new row to the current tx in case everything
is ok.

set_next_tx_row() is extracted from applier_read_tx() because it's a
common part of transaction assembly both for recovery and applier.

The only difference for recovery will be that the routine which's
responsible for tx assembly won't read rows. It'll be a callback ran on
each new row being read from WAL.

Prerequisite #5874
Part-of #5566