Eliminate COPY FREEZE use of XLOG_HEAP2_VISIBLE

Eliminate COPY FREEZE use of XLOG_HEAP2_VISIBLE

Instead of emitting a separate WAL XLOG_HEAP2_VISIBLE record for setting
bits in the VM, specify the VM block changes in the
XLOG_HEAP2_MULTI_INSERT record.

This halves the number of WAL records emitted by COPY FREEZE.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_ZMw6Npd_qm2KM%2BFwQ3cMOMx1Dh3VMhp8-V7SOLxdK9-g%40mail.gmail.com