Revert "Allow logical replication snapshots to be database-specific"

Revert "Allow logical replication snapshots to be database-specific"

This reverts commit 0d3dba38c777, which was determined to have
fundamental flaws.  This restricts REPACK (CONCURRENTLY) so that only
one process can run it concurrently on different tables and even on
different databases; we'll lift that restriction in another way during
the next development cycle.

Reported-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CAA4eK1Jg21ODQ7fS2fvN5W_S5kDRhAP5inj3XMRQaa=s-GbYhw@mail.gmail.com