Bootstrap UCX processes outside of MPI

[nirandaperera]

Parent issue: #575

• Figure out how to create a context out of locally spawned processes (determine what metadata is required to spawn processes)
• Get a UCC example running without MPI
• Spawn UCX processes outside of MPI (use Dask/ ssh+bash to do this)

Possible options for a distributed key value store

• Redis
• ZeroMQ
• Dask (cluster)

[kaiyingshan]

Process of initializing UCX context:

• get rank and world size (currently by MPI)
  init UCP context (with cylon::ucx::initContext)
  init send/recv workers (with cylon::ucx::initWorker)
• allgather ucp recv worker address (currently by MPI), and create endpoint map
• build with MPI if UCC is not present

Initializing UCC context:
get rank and world size from UCX communicator assigned by constructor

• create ucc_context with oob allgather (currently using MPI)