Subclass distributed.Adaptive

[TomAugspurger]

This is still a work in progress. I still have a few pieces that were previously all in ._adapt that need to be added back in. This depends on dask/distributed#1311

• jobs with queued_active (I think move to should_scale_up)
• len(s.workers) < len(self.cluster.workers)
• Starting up busy number of clusters, instead of 2 * the current (from distributed.Adaptive.get_scale_up_kwargs

[TomAugspurger]

@mrocklin I'm not sure what kind of backwards compatibility guarantees we make here, but distributed.Adaptive takes (scheduler, cluster), while dask_drmaa.Adaptive takes (cluster, scheduler). I think we should align on distributed's signature.

5fd79bc warns if the arguments are in the incorrect order.

[mrocklin]

Adaptive isn't used much. I encourage a move-fast-and-break-things approach.