Provide asyncio-based communication API

[jakirkham]

In Dask we are looking into using an asyncio-based communication API directly ( dask/distributed#4513 ). This would help standardize a lot of the communication in Dask. One thing that would be interesting to explore would be whether we can expose UCX communication through a similar looking (if not the same) API. This would cutdown on the amount of handling Dask does around UCX communication. Also it should help us better leverage other asyncio backends like uvloop, which Dask already has support for ( dask/distributed#4448 ) and we are already making some use of.