change box_rearrange_create() and compute_counts() to support async

[edhartnett]

Some changes are needed to box_rearrange_create() to support async.

There are 3 pio_swapm() calls, and they all take place on the union_comm, but the arrays that are used are only allocated to size num_comptasks. This is correct for non-async, but incorrect for async.

box_rearrange_create() calls compute_counts(), which also needs changes in support of async. There are two pio_swapm() calls which are called on the union_comm.

This is part of #89.