[FEA] communicator abstraction

[afender]

We should have a communicator abstraction regardless of what backend is used. This allows our code to be independent of the actual communication framework.

The communicator would have features spanning from comm.get_rank() to comm.allReduce(...)

We should start from RAFT see #523 and rapidsai/raft#7
In the long run, we should allow multiple communicators and host/device targets.

[afender]

While #840 added the C++ temporary communicator in 0.14. We should repurpose this issue to integrate RAFT comms

[afender]

done #947 #944 #896 .
Temporary comm was canceled, now using RAFT