Wake up launcher when enabling a subscription

Otherwise one would have to wait up to DEFAULT_NAPTIME_PER_CYCLE until
the subscription worker is considered for starting.

There is a small race condition:  If one enables a subscription right
after disabling it, the launcher might not have registered the stopping
when receiving the wakeup signal for the re-enabling.  The start will
then not happen right away but after the full cycle time.

Author: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>

Author: petere

src/backend/commands/subscriptioncmds.c

@@ -644,6 +644,9 @@ AlterSubscription(AlterSubscriptionStmt *stmt)
 					BoolGetDatum(enabled);
 				replaces[Anum_pg_subscription_subenabled - 1] = true;
 
+				if (enabled)
+					ApplyLauncherWakeupAtCommit();
+
 				update_tuple = true;
 				break;
 			}