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/script/Makefile.am

@@ -4,7 +4,7 @@ EXTRA_DIST =					\
 
 distributable_scripts =				\
   hydra-init					\
-  hydra-evaluator				\
+  hydra-eval-jobset				\
   hydra-server					\
   hydra-update-gc-roots				\
   hydra-s3-backup-collect-garbage		\