This change refactors find_dl_schedule() to only call dependent functions
as needed. In particular, directory_fetches_from_authorities() only needs
to be called on clients.
Stopping spurious directory_fetches_from_authorities() calls on every
download on public relays has the following impacts:
* fewer address resolution attempts, particularly those mentioned in 21789
* fewer descriptor rebuilds
* fewer log messages, particularly those limited in 20610
Fixes 23470 in 0.2.8.1-alpha.
The original bug was introduced in commit 35bbf2e as part of prop210.
