Change HarvestJobManager to submit jobs based on Harvester ready events

New strategy

We have decided to abandon the current attempt to maintain a Harvester state in the HarvestJobManager, which is very difficult to achieve in a asynchronous system. Instead we are going to try a more event based approach, which should be simpler and more robust. The job dispatching will be as follows:

1. All Harvesters sends a 'Ready for Job' message every 'JOB_STATUS_MESSAGE_SEND_INTERVAL' if it is ready to process a job. The Harvester will not sending any messages if is is running a harvest.
2. Each time the HarvestJobManager receives a 'Ready for Job' message it will check the job database for jobs ready to be run. If a ready job is found it is dispatched.

This means that:

• We don't have to have a JobMessage Scheduler running in the HarvestDispatcher.
• We don't have to have to try to maintain a Harvester state in the HarvestDispatcher.
• Harvesters don't need to send 'Not readys messages
• Harvester shutdowns don't need to be handled.

Speciel cases are:

• Shutdown of a harvester: If a Harvester stops it will stop sending 'Ready for jobs' messages, and will therefore not cause new jobs to be dispatched.
• The HarvestJobManager is restarted: The HarvestJobManager will inside of a 'JOB_STATUS_MESSAGE_SEND_INTERVAL' receive 'Ready for job' messages from all ready harvester, on will therefore quickly have send jobs to idle harvesters.
• The HarvesterJobDistapacher is slow to send a new Job, which causes the Harvester to send 2 'Ready for job' messages before a Job is received. This might cause jobs to slowly aggregate on the Job queue, so we need a mechanism so Harvesters check the job queue for any outstanding jobs before.
  So we should introduce a extra feature:

3. When a Harvester becomes ready it should wait 'TIME_TO_LISTEN_FOR_NEW_JOB_BEFORE_SENDING_READY_MESSAGE' before .....