Review Request: When sending NoMasterDetectedMessage, we need to clear the master sequence number in order to ensure we still send NewMasterDetectedMessage if the master is the same. Otherwise, we can get stuck in a master-less state.

[Ben Mahler]

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Review request for mesos, Benjamin Hindman and Vinod Kone.


Description
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This is also along the way to fixing MESOS-305.

This fixes a bug where we were not clearing the sequence number when sending a 
NoMasterDetectedMessage.
As a result, this removes the need for hiding NoMasterDetectedMessages from 
contending non-leaders.


Diffs
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  src/detector/detector.cpp 7a8355162d543e017505dd58efd2d7bf96f99623 

Diff: https://reviews.apache.org/r/10161/diff/


Testing
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make check

I'd like to add a test for this, but I don't see how to induce a timedout() 
call on the ZooKeeperMasterDetectorProcess, unless I add some plumbing to get a 
handle to the underlying process. Suggestions?


Thanks,

Ben Mahler