Ignore cached bridge descriptors until we check if they are running
Split off #24367 (moved):
[An] underlying issue here is that every use of any_bridge_descriptors_known() in tor is wrong. Instead, we want to know if num_bridges_usable() > 0.
This is a bug introduced in commit 93a8ed3 in 0.3.2.1-alpha by #23347 (moved). But it was also a bug in commit eca2a30 in 0.2.0.3-alpha, but we've never encountered it before, because we always retried our bridges immediately (and far too often).
My branch bug24367 at https://github.com/teor2345/tor.git passes all of make test-network, but fails some of the guard and microdesc unit tests. This probably means the unit tests were relying on the buggy behaviour, and need to be changed. I might need nickm or asn to help me fix the unit tests, because they wrote them.