Serve a cache hit with no database read or write (closes #309)

A hit opens the variant file and nothing else: its content type comes
from the file's first bytes, and a file in no output format is a miss.
Its access time and count, and the access time of a cached source a
miss uses, go without waiting to one background goroutine that writes
them once a second; each kind of access time waits in a channel of
10,000 that drops the rest. Expired negative cache rows are left to
CleanExpired. The .meta files, metaCache, Lookup and golang-lru go,
with their tests; the two reconciliation walks, identical without the
.meta skip, share one. make bench runs a parallel cache hit benchmark
on the host, outside make check.

Model: opus-5-5

internal/handlers/handlers.go

@@ -76,8 +76,8 @@ func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
 		OnStart: func(_ context.Context) error {
 			return s.initImageService()
 		},
-		// The pending counts are written here, before the database's stop
-		// hook closes it.
+		// The pending counts and access times are written here, before the
+		// database's stop hook closes it.
 		OnStop: func(ctx context.Context) error {
 			if s.imgCache == nil {
 				return nil
@@ -128,7 +128,8 @@ func (s *Handlers) initImageService() error {
 	// rows, which is all it does when the disk cache is disabled.
 	cache.StartEviction(imgcache.DefaultEvictionInterval)
 
-	// Writes the counts requests could not write by their deadline
+	// Writes the counts and the access times of cache hits that requests
+	// hand over, so that no request waits for the database to record them
 	cache.StartPendingCountWrites()
 
 	// Create the fetcher config