In live.go we use a semaphore to limit how many inflight signing
requests we can have, so a flood of OCSP traffic doesn't flood our CA
instances. If traffic exceeds our capacity to sign responses for long
enough, we want to eventually start fast-rejecting inbound requests that
are unlikely to get serviced before their deadline is reached. To do
that, add a MaxSigningWaiters config field to the OCSP responder.
Note that the files in //semaphore are forked from x/sync/semaphore,
with modifications to add the MaxWaiters field and functionality.
Fixes#6392
Jacob Hoffman-Andrews