riak_cs_access_archiver_manager always fail to throttle access archiver

number of active workers exceeded max_workers, because the
`handle_call({archive, _, _}, ...)` never matches to the message
called from `riak_cs_access_archiver_manager:archive/2`.

Not sure what to test here, or how to reproduce, probably I can write riak_test by setting `access_archiver_max_workers` as 0. I'd be happy
write the test if it's not trivial.