hs: Add downloaded counter to an HSDir cache entry

hs: Add downloaded counter to an HSDir cache entry

This adds a counter for the number of times a descriptor is downloaded from an
HSDir. Future commit will change the OOM subsystem to clean that cache based on
the lowest downloaded counts instead of time in cache.

In order to raise the bar even more for an attacker, the downloaded counter is
only marked when the directory request stream is closed. To pull this off, the
HS identifier on the directory connection is populated with the blinded key
requested (only on success). Finally, when the connection closes, we can then
lookup the cache entry with it and increment the counter.

Part of #40996

Signed-off-by: David Goulet <dgoulet@torproject.org>