Authorities were never sending back 503 error code because by design they
should be able to always answer directory requests regardless of bandwidth
capacity.

However, that recently backfired because of a large number of requests from
unknown source using the DirPort that are _not_ getting their 503 code which
overloaded the DirPort leading to the authority to be unable to answer to its
fellow authorities.

This is not a complete solution to the problem but it will help ease off the
load on the authority side by sending back 503 codes *unless* the connection
is from a known relay or an authority.

Fixes #33029



Signed-off-by: David Goulet <dgoulet@torproject.org>