It was observed from a log of a failed CI test that the ACK from
clearing the rally items took 6 wallclock seconds to arrive.
We were not waiting for that ACK to arrive before sending the request
for the mission item count, but if it has taken more than six seconds
for the ACK to arrive it is reasonable to assume that MISSION_COUNT
could very well take more than the 10 seconds we allow for it.
If we drain the mav before sending the request for the mission count we
should remove any signficiant timing problem due to a backlog of mavlnk
messages, but the amount of traffic here is problematic.
Also drain in lots of other places where we might be spending way too
long parsing messages.
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