Disallow starting server with insufficient wal_level for existing slot.

Previously it was possible to create a slot, change wal_level, and
restart, even if the new wal_level was insufficient for the
slot. That's a problem for both logical and physical slots, because
the necessary WAL records are not generated.

This removes a few tests in newer versions that, somewhat
inexplicably, whether restarting with a too low wal_level worked (a
buggy behaviour!).

Reported-By: Joshua D. Drake
Author: Andres Freund
Discussion: https://postgr.es/m/20181029191304.lbsmhshkyymhw22w@alap3.anarazel.de
Backpatch: 9.4-, where replication slots where introduced

src/backend/replication/logical/logical.c

@@ -74,6 +74,11 @@ CheckLogicalDecodingRequirements(void)
 {
 	CheckSlotRequirements();
 
+	/*
+	 * NB: Adding a new requirement likely means that RestoreSlotFromDisk()
+	 * needs the same check.
+	 */
+
 	if (wal_level < WAL_LEVEL_LOGICAL)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),