fixed small bug that caused unit tests to fail in certain situations
implemented state history with timestamps (for pilots only)
support for multiple agents
state history for CUs
more CU state history
added input transfers to CU history
Uncluttered a few things on the backend level.

Implemented proper "autonomous" transfer processes that don't rely on
weird data structures to pass information around. Processes now operate
atomically on MongoDB so they can be scaled up without provoking race conditions.
some commonsensical renaming
cleaned up output file worker
Added support for worker process concurrency control

UnitManager now has two optional parameters:

* input_transfer_workers (default: 1)
* output_transfer_workers (default: 1)

This allows to controll the number of I/O transfer workers. Some quick tests
shows some significant speedups in certain scenarios. Comprehensive benchmarks
would certainly helpful and insightful at some point... ;-)
support for agent selection
added non-disruptive framework for upcoming MPI stuff...
some small fixes + cleaned up UnitManager controller