Once the cache was full, every write sorted all 50,000 entries to find the
oldest. A Map keeps insertion order, so keep the map in fetch-time order
and delete from the front: the resolver re-inserts each point it refetches
(an overwrite would leave it in its old place), and the disk load sorts
once, since a file written before this change, or a migrated raw-batch
cache, is not in age order.
Suggested by asturksever on #975.
daikaginza
e60cbb42e3