Reserve bandwidth for most recently hovered link (#78362)
The prefetch scheduler limits the number of concurrent prefetch
requests, to avoid saturating browser's internal network queue. As a
starting point, I had previously hardcoded this to 3 — intentionally
stingy, to leave room for additional optimizations.
In this PR, I allow the most recently hovered link to exceed the default
limit. This increased limit only applies to the *most recently* hovered
link; if a different link is hovered, the previous one is bumped back
down to the default.
The principle is that when hovering over a new link, we should have
enough bandwidth to start prefetching its data. (In practice, because we
don't cancel in-progress requests, it's still possible that we'll fill
up the queue, but this makes it less likely.)
As part of this change, I changed the implementation of the internal
Intent priority level. Intent used to assigned to any link that was ever
hovered over; it would never get bumped back down to Default, even on
mouseexit. This was intentional at the time, but given this new
optimizaiton, it makes more sense to downgrade previously hovered links
to free up the bandwidth.
