[Buildbot] #3088: add a foreign key from builds to buildslaves

[Buildbot trac]

#3088: add a foreign key from builds to buildslaves
------------------------+----------------------
Reporter:  dustin       |      Owner:
    Type:  enhancement  |     Status:  new
Priority:  major        |  Milestone:  0.9.+
 Version:               |   Keywords:  database
------------------------+----------------------
 {{{
 #! python
     # This table contains basic information about each build.
     builds = sa.Table('builds', metadata,
 ...
                       # slave which performed this build
                       # TODO: ForeignKey to buildslaves table, named
 buildslaveid
                       # TODO: keep nullable to support slave-free builds
                       sa.Column('buildslaveid', sa.Integer),
 }}}

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Ticket URL: <http://trac.buildbot.net/ticket/3088>
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Buildbot: build/test automation