Use PostgreSQL notifications for queue events

Hydra-queue-runner now no longer polls the queue periodically, but
instead sleeps until it receives a notification from PostgreSQL about
a change to the queue (build added, build cancelled or build
restarted).

Also, for the "build added" case, we now only check for builds with an
ID greater than the previous greatest ID. This is much more efficient
if the queue is large.

src/lib/Hydra/Helper/Nix.pm

@@ -469,6 +469,8 @@ sub restartBuilds($$) {
         # FIXME: Add this to the API.
         # FIXME: clear the dependencies?
         $db->resultset('FailedPaths')->search({ path => [ @paths ]})->delete;
+
+        $db->storage->dbh->do("notify builds_restarted");
     });
 
     return scalar(@buildIds);