Make dask optional

[mpaget]

A distributed.dask Client is started for each call to PolygonDrill/PixelDrill.query_handler(). This may not work so well for multiple async workers on a node.

Make dask an option. May need if dask: do dask things; else: do standard things paths in the few places where dask is assumed.

Dask is assumed in datacube.virtual so but should be ok if dask_chunks=None is passed down.