Specifically, I want to be able to post a bunch of things and wait on all of them to complete. Something like:

for (int i = 0; i < 10; i++) margo_post_forward(mid, handles[i], &in[i], &ops[i]);
margo_waitall(10, ops);
for (int i = 0; i < 10; i++) { assert(ops[i].ret == HG_SUCCESS); }

Not needed right this moment, but would be useful in the future.