added margo_deregister

See merge request !6

margo_finalize now doesn't clean up until pending operations are completed

See merge request sds/margo!5

refactor provider multiplexing

See merge request sds/margo!4

so that master increments when this is merged

- 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

- don't retrieve current time in primary code path unless we know there
  are timers to check

mercury does not reset the target_id as part of HG_Reset

added shutdown feature

See merge request sds/margo!3

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