Concurrent hydra-evaluator

This rewrites the top-level loop of hydra-evaluator in C++. The Perl
stuff is moved into hydra-eval-jobset. (Rewriting the entire evaluator
would be nice but is a bit too much work.) The new version has some
advantages:

* It can run multiple jobset evaluations in parallel.

* It uses PostgreSQL notifications so it doesn't have to poll the
  database. So if a jobset is triggered via the web interface or from
  a GitHub / Bitbucket webhook, evaluation of the jobset will start
  almost instantaneously (assuming the evaluator is not at its
  concurrency limit).

* It imposes a timeout on evaluations. So if e.g. hydra-eval-jobset
  hangs connecting to a Mercurial server, it will eventually be
  killed.

src/sql/hydra.sql

@@ -83,6 +83,19 @@ create function notifyJobsetSharesChanged() returns trigger as 'begin notify job
 create trigger JobsetSharesChanged after update on Jobsets for each row
   when (old.schedulingShares != new.schedulingShares) execute procedure notifyJobsetSharesChanged();
 
+create function notifyJobsetsAdded() returns trigger as 'begin notify jobsets_added; return null; end;' language plpgsql;
+create trigger JobsetsAdded after insert on Jobsets execute procedure notifyJobsetsAdded();
+
+create function notifyJobsetsDeleted() returns trigger as 'begin notify jobsets_deleted; return null; end;' language plpgsql;
+create trigger JobsetsDeleted after delete on Jobsets execute procedure notifyJobsetsDeleted();
+
+create function notifyJobsetSchedulingChanged() returns trigger as 'begin notify jobset_scheduling_changed; return null; end;' language plpgsql;
+create trigger JobsetSchedulingChanged after update on Jobsets for each row
+  when (((old.triggerTime is distinct from new.triggerTime) and (new.triggerTime is not null))
+        or old.checkInterval != new.checkInterval
+        or old.enabled != new.enabled)
+  execute procedure notifyJobsetSchedulingChanged();
+
 #endif