holy crap -- multiprocessing !

still experimental but it's getting there. required
for a whole host of things, like file transfer but
also asynchronous operations.

somewhat nightmarish to implement, but seems to work quite
fine so far.
Removed Attribute interface as it breaks garbage collection.

Without inheriting from Attribute, ComputePilot and PilotManager
destructors (__del__) are called now as expected.
Much simpler scheduler selector sans plugin manager.

The plugin manager seems to break Python garbage collection
even more. Reliable destructors are crucial for SP, hence
this radical simplification.
session destructor finally working!
small sanity check at session 'tor'
small improvement
cleanup towards data transfer
persistent sessions and credentials
working worker thread...
fixed worker reconnect
state_details -> log
added some tests