Save retrieved bridge information in our state file

Currently, Tor clients don't remember anything about bridges after a restart. So for example, if there was a torrc Bridge line without a fingerprint specified, and Tor retrieves the fingerprint from the descriptor, it won't save it on the disk.

Or, if a bridge changed IP:PORT, but we managed to get its new ip:port using the bridge authority (see #12599 (moved)), we will not save this down either.

Fortunately, Sebastian wrote proposal192 about this issue: https://gitweb.torproject.org/torspec.git/tree/proposals/192-store-bridge-information.txt

We should re-read the proposal, revise it as needed and implement it.