Define format for per-mirror hostname
Note that all mirrors will need to answer both on the fallback hostname
dl.amnesia.boum.org) and on their own one, so that we can
add them to the fast and reliable fallback pool (for non-JS users)
without any need for server-side configuration changes.
Also note that whatever per-mirror hostname format we choose must exist, and be maintained, in the DNS.
Feature Branch: doc/11054-per-mirror-hostname
Parent Task: #7161 (closed)