Aggregate state properly reset on commitEvents()

There is a possible issue when apply invocations are nested and there
 is an aggregate cache in place. The cached aggregate could be kept
 in an inconsistent state.

Issue #AXON-268 Fixed
The DisruptorCommandBus can now handle commands not affecting aggregates

When no aggregate is loaded, nor added to the repository, the
DisruptorCommandBus would fail with a NullPointerException. It has
now been fixed to cope with this situation as expected.

Issue #AXON-267 Fixed