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.

tools/varnishqueue/nagios_check.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python
+
+import sys
+import psycopg2
+from datetime import timedelta
+
+# Up to 5 minutes delay is ok
+WARNING_THRESHOLD=timedelta(minutes=5)
+# More than 15 minutes something is definitely wrong
+CRITICAL_THRESHOLD=timedelta(minutes=15)
+
+if __name__ == "__main__":
+	if len(sys.argv) != 2:
+		print "Usage: nagios_check.py <dsn>"
+		sys.exit(1)
+
+	conn = psycopg2.connect(sys.argv[1])
+	curs = conn.cursor()
+
+	# Get the oldest entry that has not been completed, if any
+	curs.execute("SELECT max(now()-added) FROM varnishqueue.queue WHERE completed IS NULL")
+	rows = curs.fetchall()
+	conn.close()
+
+	if len(rows) == 0:
+		print "OK, queue is empty"
+		sys.exit(0)
+
+	age = rows[0][0]
+
+	if age < WARNING_THRESHOLD:
+		print "OK, queue age is %s" % age
+		sys.exit(0)
+	elif age < CRITICAL_THRESHOLD:
+		print "WARNING, queue age is %s" % age
+		sys.exit(1)
+	else:
+		print "CRITICAL, queue age is %s" % age
+		sys.exit(2)