Fixed issue where some aggregate would not be correctly created

This would occur when two sagas of different types, managed by the same
saga manager and triggered by the same association value are created at the
same time.

Issue #AXON-204 Fixed
Fixed issue where command callback is not notified of a missing handler

Sending an unroutable message on the SimpleCommandBus would throw an exception,
but would not notify the callback, as defined by the interface. In certain
configurations, this could lead to silent errors.

Issue #AXON-203 Fixed