Replace pgq with trivial local queue implementation

Replace pgq with trivial local queue implementation

The queue used for varnish purges has so few entries that it's really
not worth paying the management overhead for skytools/pgq. Instead we
can use a very simple local deamon using LISTEN/NOTIFY to fire them off.

Now include a proper nagios plugin in this package, so we can get rid
of the not-very-nice munin plugin currently used in the deployment.