Add some asserts based on LWLockHeldByMe() for replication slot statistics

Add some asserts based on LWLockHeldByMe() for replication slot statistics

Two assertions checking that ReplicationSlotAllocationLock is acquired
are added to pgstat_create_replslot() and pgstat_drop_replslot(),
corresponding to the routines in charge of the creation and the drop of
replication slot statistics.  The code previously relied on this
assumption and documented it in comments, but did not enforce this
policy at runtime.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/Ze_p-hmD_yFeVYXg@paquier.xyz