Ensure our mirror pool is ready for temporary surge of downloads

Problem: assume many people suddenly want to try Tails for the first time, e.g. after a successful press article; given we distribute the download load equally over almost all our mirrors, that have vastly different capacity, chances are that the slowest ones will quickly get overloaded ⇒ downloads are super slow or fail.

To solve this, we need:

• either many more mirrors, but even if we can find them easily, then it would be a PITA to maintain the pool
• or spread the load according to the capacity of our current mirrors: benchmark all mirrors, and adjust their weight according to the download bandwidth they can provide.

Parent Task: #12406

Related issues

• Related to #7864
• Has duplicate #14604 (closed)

Original created by @intrigeri on 12407 (Redmine)