dir-auth: Dir auths should resume sending 503's but never to relays or other dir auths

This is a child ticket from legacy/trac#33018 (moved).

The idea here is that even if we hit the global write limit (bw), we should not return 503 code but rather answer another directory authority.

Dirauth must be able to talk to each other at all time regardless of the bandwidth state.

Setting 043 milestone because this should be considered a bug and could even be considered for backport since dirauth are suffering from this at the moment.