Turn hydra-notify into a daemon

It now receives notifications about started/finished builds/steps via
PostgreSQL. This gets rid of the (substantial) overhead of starting
hydra-notify for every event. It also allows other programs (even on
other machines) to listen to Hydra notifications.

src/hydra-queue-runner/queue-monitor.cc

@@ -193,13 +193,12 @@ bool State::getQueuedBuilds(Connection & conn,
                     (build->id)
                     ((int) (ex.step->drvPath == build->drvPath ? bsFailed : bsDepFailed))
                     (time(0)).exec();
+                notifyBuildFinished(txn, build->id, {});
                 txn.commit();
                 build->finishedInDB = true;
                 nrBuildsDone++;
             }
 
-            enqueueNotificationItem({NotificationItem::Type::BuildFinished, build->id});
-
             return;
         }
 
@@ -230,13 +229,12 @@ bool State::getQueuedBuilds(Connection & conn,
             time_t now = time(0);
             printMsg(lvlInfo, format("marking build %1% as succeeded (cached)") % build->id);
             markSucceededBuild(txn, build, res, true, now, now);
+            notifyBuildFinished(txn, build->id, {});
             txn.commit();
             }
 
             build->finishedInDB = true;
 
-            enqueueNotificationItem({NotificationItem::Type::BuildFinished, build->id});
-
             return;
         }