Fix races between deactivation of logical decoding and slot creation.

Fix races between deactivation of logical decoding and slot creation.

On standbys, logical decoding can be deactivated while a logical slot
is being created: either by replaying an
XLOG_LOGICAL_DECODING_STATUS_CHANGE record, or by the end-of-recovery
transition upon promotion, which deactivates logical decoding if no
valid logical slot exists. Both could interleave with a check of the
logical decoding status performed before creating a new slot because
the slot invalidation executed as part of the deactivation cannot find
a slot being created.

For regular slot creation on standbys, EnsureLogicalDecodingEnabled()
assumed that logical decoding must still be enabled during recovery
since the caller had already checked it, tripping an assertion failure
if a concurrent deactivation interleaved.

For slot synchronization, the local slot could be created and
persisted based on the remote slot information fetched before the
deactivation was replayed, leaving a valid slot whose restart_lsn
precedes the deactivation.

Fix both paths by re-checking the logical decoding status after the
new slot has been created: regular slot creation raises an error, and
slot synchronization skips persisting the slot. If the deactivation
happens after the re-check instead, it is guaranteed to invalidate the
newly created slot.

Reviewed-by: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/CAD21AoDEB99VtNbQdDrNd=1gQupJNGMfW_5kdnxq03Q82EK3ag@mail.gmail.com
Backpatch-through: 19