Fix a race that can cause hydra-queue-runner to ignore newly added builds

As @dtzWill discovered, with the concurrent hydra-evaluator, there can
be multiple active transactions adding builds to the database. As a
result, builds can become visible in a non-monotonically increasing
order, breaking the queue monitor's assumption that build IDs only go
up.

The fix is to have hydra-eval-jobset provide the lowest build ID it
just added in the builds_added notification, and have the queue
monitor check from there.

Fixes #496.

src/sql/hydra.sql

@@ -233,9 +233,6 @@ create table Builds (
 
 #ifdef POSTGRESQL
 
-create function notifyBuildsAdded() returns trigger as 'begin notify builds_added; return null; end;' language plpgsql;
-create trigger BuildsAdded after insert on Builds execute procedure notifyBuildsAdded();
-
 create function notifyBuildsDeleted() returns trigger as 'begin notify builds_deleted; return null; end;' language plpgsql;
 create trigger BuildsDeleted after delete on Builds execute procedure notifyBuildsDeleted();