Point-2-Point bootstrap for UCX Backend

[sbahirnv]

Currently, UCX Backend of Realm's Network Module is bootstrapped using MPI. MPI bootstrap also implements a set of collectives used by Realm and UCX Backend itself.

We want to add new point-2-point bootstrap option for Network Module when MPI is not an option.
This new option will use known IP:PORT of each member processes to bootstrap the UCX back-end of network module. It will not implement the collectives.

The collectives will be implemented in the UCX Backend itself using UCC.