Enable threading for dispatching non-DB jobs #9

In some more curious situations, it may be desirable to run some threads in the backend for managing multiple I/O jobs.

Current gating aspects:

• Threads cannot have access to the database (should be easy with existing protections)
• Parts of pg-python rely on global state, Py_ALLOCATE_OWNER()
• Thread finalization on_proc_exit?
• Other lower level problems/threads in the backend are frickin' scary?

Another option is to just use event driven async systems.