With proxies sharing a database, a batch can flush after a newer hit is
already written, and overwriting last_accessed_at and updated_at with
its older time could reorder LRU eviction. Both now keep the later of
the stored and incoming times. The count still adds up.
Every cached download wrote its hit in its own UPDATE. SQLite runs on
one connection, so each download was a write transaction the rest
queued behind, holding a warm npm cache near 1,000 requests a second
with the CPU two thirds idle.
Hits are now counted in memory and written in one transaction every
database.hit_flush_interval, "1s" by default, "0" for a write per hit.
A failed write keeps the hits for the next flush, and Close writes
what is pending. The same test then served about 2,200 requests a
second.
Closes#322.
