Without this change, if we have a working bridge, and we add a new bridge, we will schedule the fetch attempt for that new bridge descriptor for three hours(!) in the future.
This change is especially needed because of bug #40396 (closed), where if you have one working bridge and one bridge whose descriptor you haven't fetched yet, your Tor will stall until you have successfully fetched that new descriptor -- in this case for hours.
In the old design, we would put off all further bridge descriptor fetches once we had any working bridge descriptor. In this new design, we make the decision per bridge based on whether we successfully got its descriptor.
To make this work, we need to also call learned_bridge_descriptor() every time we get a bridge descriptor, not just when it's a novel descriptor.
Fixes bug 40396.
Also happens to fix bug 40495 (redundant descriptor fetches for every bridge) since now we delay fetches once we succeed.
A side effect of this change is that if we have any configured bridges that aren't working, we will keep trying to fetch their descriptors on the modern directory retry schedule -- every couple of seconds for the first half minute, then backing off after that -- which is a lot faster than before.