Update the DNS round-robin mirror pool to only include a few fast and reliable HTTP mirrors

Originally created by @intrigeri on #11284 (Redmine)

This includes updating our internal doc (in mirrors.git) accordingly:

stop unconditionally adding new mirrors to the DNS round-robin pool;
how to manage the DNS round-robin pool (what mirrors should go in or not, when to remove mirrors from it), and keep it in sync’ with what we have in mirrors.json (e.g. when we disable a mirror in mirrors.json, presumably we also want to remove it from the DNS round-robin pool.

Edited May 15, 2020 by intrigeri

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