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decoupled DEFINE_MARGO_RPC_HANDLER into multiple macros internally so the internal macros can be used in exotic cases, like in Thallium

- first draft, not complete

- proposed by Tommaso Tocci 
- follows the naming convention of the Mercury API
- retains existing margo_init() for compatibility

- 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