rust-status: cap the number of concurrent threads to 16
During benchmarking it was determined that the use of more threads is very
advantageous... until we use more than 16. This is most likely due to some
resource contention (thrashing, etc.). Until we have time to figure out and
fix the underlying cause, let's just cap at 16 threads.
Differential Revision: https://phab.mercurial-scm.org/D12384