- mplex_id terminology no lonter in api, replaced by provider_id
- no longer uses target_id hook in Mercury API, but instead multiplexes
  into underlying hg_id_t
- adds provider_id argument to margo_forward

This is untested; examples and test programs have not been updated to
use it yet.

- See #40

margo now allow ABT_POOL_NULL for multiplexing, and allow to associate a new pool even for multiplex id 0

added margo functions and macro to register RPCs and avoid using a global mapping between hg_context and margo instances