The `Cancelled` struct now reflects multiple potential reasons
for a query execution to be cancelled, including unexpected cycles.
It also gives information about the participants that lacked
recovery information.
Before we could not observe the case where:
* thread A is blocked on B
* cycle detected in thread B
* some participants are on thread A and have to be marked
(In particular, I commented out some code that seemed necessary and
didn't see any tests fail)
