Be sure to release LogicalRepLauncherLock in DROP SUBSCRIPTION command.

Previously DROP SUBSCRIPTION command forgot to release the lock at all.

Original patches by Kyotaro Horiguchi and Michael Paquier,
but I didn't use them.
Discussion: http://postgr.es/m/20170201.173623.66249355.horiguchi.kyotaro@lab.ntt.co.jp

src/backend/commands/subscriptioncmds.c

@@ -514,6 +514,8 @@ DropSubscription(DropSubscriptionStmt *stmt)
 	/* Kill the apply worker so that the slot becomes accessible. */
 	logicalrep_worker_stop(subid);
 
+	LWLockRelease(LogicalRepLauncherLock);
+
 	/* Remove the origin tracking if exists. */
 	snprintf(originname, sizeof(originname), "pg_%u", subid);
 	originid = replorigin_by_name(originname, true);