Improve consistency of replication slot statistics

The replication slot stats stored in shared memory rely on an internal
index number.  Both pgstat_reset_replslot() and pgstat_fetch_replslot()
lacked some LWLock protections with ReplicationSlotControlLock while
operating on these index numbers.  This issue could cause these two
functions to potentially operate on incorrect slots when taken in
isolation in the event of slots dropped and/or re-created concurrently.

Note that pg_stat_get_replication_slot() is called once per slot when
querying pg_stat_replication_slots, meaning that the stats are retrieved
across multiple ReplicationSlotControlLock acquisitions.  So, while this
commit improves more consistency, it may still be possible that
statistics are not completely consistent for a single scan of
pg_stat_replication_slots under concurrent replication slot drop or
creation activity.

The issue should unlikely be a problem in practice, causing the report
of inconsistent stats or or the stats reset of an incorrect slot, so no
backpatch is done.

Author: Bertrand Drouvot
Reviewed-by: Heikki Linnakangas, Shveta Malik, Michael Paquier
Discussion: https://postgr.es/m/ZeGq1HDWFfLkjh4o@ip-10-97-1-34.eu-west-3.compute.internal

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