GitLab

- this doesn't cause a warning and makes the prototype a little less
  confusing now in conjunction with previous commit

- 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*

- 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