Potentially temporarily change the barrier in the ORTE DPM component
to be based on grpcomm (i.e., an out-of-band based barrier) rather
than the simplistic PML-based barrier that it currently uses.

This is pending a larger discussion with Nathan and George, but it
will allow the usnic BTL to stop assert()-failing in light of the
recent del_procs() change.
usnic: re-order teardown sequence now that del_procs() is actually called

Now that the infrastructure is calling BTL del_procs() before the BTL
finalize(), the usnic BTL had to re-order some of its teardown
sequence to avoid assert() failing.

This is part of a larger conversation involving #4669.  Since
MPI_FINALIZE and MPI_COMM_DISCONNECT currently use an
oob/grpcomm-based barrier, the usnic BTL can ''absolutely know'' that
these endpoints and procs will no longer be used.  If the ORTE DPM
goes back to a PML-based barrier, the usnic BTL will need to grow more
complex teardown semantics (a la TCP socket FIN/ACK/FIN_WAIT states).

Refs #4669