Make sure bridges are definitely running before delaying directory fetches

Retry directory downloads when we get our first bridge descriptor
during bootstrap or while reconnecting to the network. Keep retrying
every time we get a bridge descriptor, until we have a reachable bridge.

Stop delaying bridge descriptor fetches when we have cached bridge
descriptors. Instead, only delay bridge descriptor fetches when we
have at least one reachable bridge.

Fixes bug 24367; bugfix on 0.2.0.3-alpha.