Implement a virtual clock on the FlightContextFactory

[corstian]

In order to properly dispose expired context instances to spare memory in long running processes, and properly fire triggers, bot in real-time situations as well as batch processing, we'd need a time stamp which identifies "current" time.

A major side effect is that we're unable to fire a completed event when an (arrival) theory has expired. This might happen in cases where an aircraft lands, and no further transponder messages are being received. The flight context will be discarded, and the theory will be discarded resulting in an incomplete flight record.