[Buildbot] #2818: [nine] rewrite buildsteps to not write to status

[Buildbot trac]

#2818: [nine] rewrite buildsteps to not write to status
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Reporter:  dustin       |       Owner:  dustin
    Type:  enhancement  |      Status:  new
Priority:  major        |   Milestone:  0.9.0
 Version:  0.8.7p1      |  Resolution:
Keywords:               |
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Comment (by dustin):

 This is almost ready for a pull req, with the rest to be done in branch
 2818b.  I just need to test `BuildStep.getCurrentSummary` and
 `getResultSummary`.

 Once that's done, in the new branch:

 * remove progress/ETA support (I think there's already a bug to re-add
 this in a more functional fashion)
 * add data API support for hidden steps
 * remove master/buildbot/status/buildstep.py, moving it to
 buildbot.util.pickle
 * [WIP] convert stepStateStrings to take only a single string, since
 that's all we produce with the summary handling

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Ticket URL: <http://trac.buildbot.net/ticket/2818#comment:3>
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