in cases where margo_forward_timed calls timeout, margo will
return HG_TIMEOUT rather than HG_CANCELED

- 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*

- first test is for the sleep functionality

we need to remember whether a sleeping thread is in the same pool
as the progress thread to properly decrement the number of
waiting ULTS.

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

- also a new version of utlist.h with APPEND_ELEM macro