QD Merlin Medipix: handle aborted/stopped acquisitions

[sk1p]

On the data channel, we don't get an indication if an acquisition has been aborted/stopped. We may need to add a timeout, and stop receiving if there is a gap of more than X seconds.

If we receive another acquisition header instead of a frame, we might be able to stop the current acquisition, and seamlessly start the next one. This would be needed if an acquisition is stopped and very soon after the next acquisition is started.