Design improvements metatask

[cboulay]

• PacketHandlerThread should start as soon as NSPDevice is created and can be the owner of the receiver queue
• PacketHandlerThread -- double check continue conditions as self._continue is unused.
• CerebusDatagramThread should open the socket with SO_REUSEADDR
• Finish work in central_shmem branch to auto-select CerebusShmemThread as ._io_thread if Central running on same system