HPS not updated in PassiveDM if a Prepare is removed before it is locally-satisfied

That may happen at Replica at Commit. E.g.

1. topology: {A, R1, R2}
2. A receives Prepare{s:1, Level:PersistToMajority} and sends it to R1/R2
3. s:1 persisted and ack'ed by A and R1
4. A sends Commit{s:1} to R1/R2
5. R2 receives Commit{s:1} and removes Prepare{s:1} from PassiveDM
6. Prepare{s:1} persisted on R2
7. the flusher calls back to PassiveDM for updating the HPS
8. the update is skipped because the tracked Container in PassiveDM is empty

Note that in PassiveDM the HPS is of type:

struct DurabilityMonitor::Position {

    Container::iterator it;

    WeaklyMonotonic<int64_t, ThrowExceptionPolicy> lastWriteSeqno{0};

}

While Position::it can point to Container::end (which is expected in the case described), Position::lastWriteSeqno must always be set to the right seqno.