It now receives notifications about started/finished builds/steps via
PostgreSQL. This gets rid of the (substantial) overhead of starting
hydra-notify for every event. It also allows other programs (even on
other machines) to listen to Hydra notifications.

* 'eval_started' has the format '<tmpId>\t<project>\t<jobset>'.

* 'eval_failed' has the format '<tmpId>'. (The cause of the error can
  be found in the database.)

* 'eval_added' has the format '<tmpId>:<evalId>'.

Plugins are now disabled at startup time unless there is some relevant
configuration in hydra.conf. This avoids hydra-notify having to do a
lot of redundant work (a lot of plugins did a lot of database queries
*before* deciding they were disabled).

Note: BitBucketStatus users will need to add 'enable_bitbucket_status
= 1' to hydra.conf.

In particular, doing a 'select * from Jobsets where ...' must be
avoided, because the 'errormsg' column can be very big.

Turn hydra-notify into a daemon

May 15 09:20:10 chef hydra-queue-runner[27523]: Hydra::Plugin::GitlabStatus=HASH(0x519a7b8)->buildFinished: Can't call method "value" on an undefined value at /nix/store/858hinflxcl2jd12wv1r3a8j11ybsf6w-hydra-0.1.2629.89fa829/libexec/hydra/lib/Hydra/Plugin/GitlabStatus.pm line 57.

(cherry picked from commit 438ddf5)