DDS15 — Cancel coherent update

Problem:
It is possible that a transaction needs to be canceled, for example because one of the participating writers gets blocked and finally stops while returning a timeout. This might lead to the situation in which you want to cancel all preceding writes as well.

Solution:
To be able to cancel such a transaction, we propose to add an additional operation like e.g. cancel_coherent_changes would then remove all samples that have already been written since the last begin_coherent_changes.