new mode that only fetches bridge descs as needed
Add a new config option FetchBridgeDescsJIT to fetch bridge descriptors
"just in time" for the bridges that Tor plans to use.
Now if you have a large bridge list (such as when using Tor Browser's
built-in bridges) or you are using transports like Snowflake where each
descriptor fetch is an expensive operation, you will only reach out to
the bridges whose descriptors you actually need.
Fixes bug 40578; bugfix on 0.2.0.3-alpha.