docs: def getOldestRequestTime(self):
          """Returns the submitted_at of the oldest unclaimed build request for
          this builder, or None if there are no build requests.
  
          @returns: datetime instance or None, via Deferred
          """
          bldrid = yield self.getBuilderId()
          unclaimed = yield self.master.data.get(
              ('builders', bldrid, 'buildrequests'),
              [resultspec.Filter('claimed', 'eq', [False])],
              order=['submitted_at'], limit=1)
          if unclaimed:
              return unclaimed[0]['submitted_at']