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

- the mercury hg_handle_t is really a pointer that references an
  internally defined structure, we can (and should, for proper
  reference counting purposes) pass it directly to ULTs without copying
- code using margo must be updated accordingly, handler ULT argument is
  an hg_handle_t now, not an hg_handle_t*

I factored the sorted queue implementation Phil created for
monitoring timeouts out into separate files (margo-timer.c/h).
There is now an interface for submitting timers that define
a callback function that's executed when the timers expire.
margo_thread_sleep() and margo_forward_timed() will both use
this interface, though only margo_thread_sleep() is implemented
at the moment.

- na_ and NA_ types and functions no longer necessary

- Demonstrated in test program.  This function allows caller to suspend
  until some other entity calls margo_finalize(). If called by a ULT
  that is executing in the same pool as the HG progress thread, then it
  will indicate to the progress thread that it does not need to yield
  execution.
- fixes #6