Make sure to disable sync rep when initializing a primary. (#801)

The step to reach the SINGLE state could be done from a pre-existing PGDATA,
such as taken from a backup tool. In that case, the postgresql.auto.conf
might have been installed with a setting that enables synchronous
replication.

Trouble is, we're initializing the first node in the system, and when we
reach the SINGLE state any setup for sync-rep does not make sense, we know
we don't have any secondary available.

src/bin/pg_autoctl/fsm_transition.c

@@ -200,6 +200,19 @@ fsm_init_primary(Keeper *keeper)
 		return false;
 	}
 
+	/*
+	 * When dealing with a pg_autoctl create postgres command with a
+	 * pre-existing PGDATA directory, make sure we can start the cluster
+	 * without being in sync-rep already. The target state here is SINGLE
+	 * after all.
+	 */
+	if (!fsm_disable_replication(keeper))
+	{
+		log_error("Failed to disable synchronous replication in order to "
+				  "initialize as a primary, see above for details");
+		return false;
+	}
+
 	/*
 	 * FIXME: In the current FSM, I am not sure this can happen anymore. That
 	 * said we might want to remain compatible with initializing a SINGLE from