[Buildbot] #3014: Add a migrator that reverts db version 40

[Buildbot trac]

#3014: Add a migrator that reverts db version 40
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Reporter:  Ben       |       Owner:
    Type:  defect    |      Status:  new
Priority:  critical  |   Milestone:  0.9.0
 Version:  master    |  Resolution:
Keywords:            |
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Comment (by Xavier Delannoy <xdelannoy@…>):

 In [changeset:"5c2cf49011e795c40745f04f39501b9d64eddf83"]:
 {{{
 #!CommitTicketReference repository=""
 revision="5c2cf49011e795c40745f04f39501b9d64eddf83"
 Trac #3014 Add a migrator that reverts db version 40
  - This commit reintroduce db version 40, and the db/test code which used
 it
 }}}

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Ticket URL: <http://trac.buildbot.net/ticket/3014#comment:4>
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