[Buildbot] #3213: Only send messages from Data API update methods

[Buildbot trac]

#3213: Only send messages from Data API update methods
------------------------+-------------------
Reporter:  dustin       |      Owner:
    Type:  enhancement  |     Status:  new
Priority:  major        |  Milestone:  0.9.0
 Version:  master       |   Keywords:  mq
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 We currently have messages sent by

  * `BuildRequest.cancelBuildRequest`
  * `BuildRequestDistributor._maybeStartBuildsOnBuilder`

 and these should move to data API update methods.

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Ticket URL: <http://trac.buildbot.net/ticket/3213>
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