[Buildbot] #3498: Nine nextSlave API change

[Buildbot trac]

#3498: Nine nextSlave API change
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Reporter:  gracinet   |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  0.9.+
 Version:  0.9.0b7    |   Keywords:
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 Apparently ```nextSlave``` API changed for Nine, and that's not a problem
 per se

 This can be really puzzling for migrated instances, because

 * errors is silently swallowed
 * it is not mentioned in
 http://docs.buildbot.net/latest/manual/installation/nine-upgrade.html

 The result for a migrated instance is that buildrequests will be added,
 yet no actual build will start, no error shows up anywhere. It's not been
 easy for me to figure that out (eliminating network issues, db issues,
 then heavy use of pdb in ```buildconfigdistributor.py```

 Seen on 0.9b7, code in current master is identical

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Ticket URL: <http://trac.buildbot.net/ticket/3498>
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