[Bug] Can't update any endpoint

[woodsaj]

see https://github.com/raintank/worldping-app/issues/113

[woodsaj]

{"message":"2016/05/26 17:29:35 [monitor.go:88 UpdateMonitor()] [E] Failed to update monitor: Invalid Collector specified for Monitor"

There is a collector assigned to this check that has been deleted. (collector_id: 101)

need to

• [x] clean up the DB
• [ ] extend the deleteCollector DB transaction to also remove entries form the monitor_collector table.