[Buildbot] #3209: Improve buildslave reconfiguration

[Buildbot trac]

#3209: Improve buildslave reconfiguration
------------------------+--------------------
Reporter:  dustin       |      Owner:
    Type:  enhancement  |     Status:  new
Priority:  minor        |  Milestone:  0.9.+
 Version:  master       |   Keywords:  simple
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 As of https://github.com/buildbot/buildbot/pull/1544, when a master is
 reconfigured it un-configures *all* of its buildslaves, then reconfigures
 the new set.  This means extra un- and re-configuration of buildslaves
 where nothing has changed, so it's quite a bit of extra noise.

 The fix for this is to calculate exactly what has changed, and only un-
 configure and configure the removed and added buildslaves, respectively.

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Ticket URL: <http://trac.buildbot.net/ticket/3209>
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