[Buildbot] #3263: Make ClusteredScheduler AsyncService

[Buildbot trac]

#3263: Make ClusteredScheduler AsyncService
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Reporter:  tardyp  |      Owner:
    Type:  defect  |     Status:  new
Priority:  major   |  Milestone:  0.9.0
 Version:  master  |   Keywords:
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 ClusteredScheduler not being AsyncService means we cannot create reliable
 integration tests based on AnyBranchScheduler.

 We should make it Async, and make sure that startService return a deferred
 calling when the scheduler is ready.

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Ticket URL: <http://trac.buildbot.net/ticket/3263>
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Buildbot: build/test automation