Stop table sync workers when subscription relation entry is removed

Stop table sync workers when subscription relation entry is removed

When a table sync worker is in waiting state and the subscription table
entry is removed because of a concurrent subscription refresh, the
worker could be left orphaned.  To avoid that, explicitly stop the
worker when the pg_subscription_rel entry is removed.

Reported-by: Masahiko Sawada <sawada.mshk@gmail.com>