revocation: Block only one thread per URL after a previous CRL fetch failed

revocation: Block only one thread per URL after a previous CRL fetch failed

If a CRL server is unresponsive, all threads trying to fetch this CRL will
block execution. If a recent previous attempt to fetch the CRL failed, it
is likely that it will fail again. While it makes sense to retry fetching
the CRL on demand with one thread, it hardly does to block additional
threads while the first one is blocked during the fetch.

So remember the timestamp of the last CRL fetch failure per URL, and do
not block more than one thread in the CRL fetch for some time. This time
is a multiple of the configured fetch timeout, so that it works well for
any configured value. With the default configuration, a failing CRL fetch
will impact concurrent CRL fetches for the same URL for 30s.