State save in different process

[dnephin]

Currently state is serialized in the same process as the twisted event loop. This should be done in another process. The state serialization process will need the following api:

• configure - reconfigure the state serialization implementation
• save - save some state
• restore - restore state for jobs/services
• shutdown - shutdown the process and implementation

This could be done with the multiprocessing module by opening a process before entering the twisted eventloop, or by creating a twisted.ProcessProtocol.

[dnephin]

Part of this should be to split Jobrun out, and make it a top level entity that can be saved with its own key (currently only job and service are top level entities). This would greatly reduce the amount of data that is serialized. It will probably make state restore a little slower, but that should be acceptable.

Additionally, it would allow us to save more job history.