Fix oldest xmin and LSN computation across repslots after advancing

Advancing a replication slot did not recompute the oldest xmin and LSN
values across replication slots, preventing resource removal like
segments not recycled at checkpoint time.  The original commit that
introduced the slot advancing in 9c7d06d never did the update of those
oldest values, and b0afdca removed this code.

This commit adds a TAP test to check segment recycling with advancing
for physical slots, enforcing an extra segment switch before advancing
to check if the segment gets correctly recycled after a checkpoint.

Reported-by: Andres Freund
Reviewed-by: Alexey Kondratov, Kyptaro Horiguchi
Discussion: https://postgr.es/m/20200609171904.kpltxxvjzislidks@alap3.anarazel.de
Backpatch-through: 11

src/backend/replication/slotfuncs.c

@@ -621,6 +621,13 @@ pg_replication_slot_advance(PG_FUNCTION_ARGS)
 	values[0] = NameGetDatum(&MyReplicationSlot->data.name);
 	nulls[0] = false;
 
+	/*
+	 * Recompute the minimum LSN and xmin across all slots to adjust with the
+	 * advancing potentially done.
+	 */
+	ReplicationSlotsComputeRequiredXmin(false);
+	ReplicationSlotsComputeRequiredLSN();
+
 	ReplicationSlotRelease();
 
 	/* Return the reached position. */