As we can't use SOCK_SEQPACKET over TCP, we now have to provide message
boundaries ourselves. We do this by appending a 16-bit length header to each
sent duplicate identity.
This ensures that no threads are active when plugins and the rest of the
daemon are unloaded.
callback_job_t was simplified a lot in the process as its main
functionality is now contained in processor_t. The parent-child
relationships were abandoned as these were only needed to simplify job
cancellation.