hydra-queue-runner: Make build notification more reliable

Previously, when hydra-queue-runner was restarted, any pending "build
finished" notifications were lost. Now hydra-queue-runner marks
finished but unnotified builds in the database and uses that to run
pending notifications at startup.

src/sql/upgrade-55.sql

@@ -0,0 +1,3 @@
+alter table Builds add column notificationPendingSince integer;
+
+create index IndexBuildsOnNotificationPendingSince on Builds(notificationPendingSince) where notificationPendingSince is not null;