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 hit count, and the access time of a cached source a
miss uses, go without waiting to the goroutine that wrote late counts,
which now writes them all once a second; each kind of access time waits
in a channel of 10,000 that drops the rest. checkNegativeCache leaves
expired rows to CleanExpired. The .meta files, metaCache, Lookup and
golang-lru go, with their tests. Dropping the .meta skip made the
variant and source reconciliation walks identical, so they share one.
make bench runs a parallel cache hit benchmark on the host, outside
make check.

Model: opus-5-5

script/bench

@@ -0,0 +1,19 @@
+#!/bin/sh
+# script/bench: run the Go benchmarks of internal/handlers, which serve
+# cache hits in parallel through the image handler and report the hits
+# served per second. A benchmark, not a check: script/check does not run
+# it. It runs Go on the host, as script/loadtest does, and compiles pixa
+# there with CGO, so it needs the C compiler and libraries that
+# `script/bootstrap --cgo` installs. -run '^$' runs no test, only the
+# benchmarks.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    CGO_ENABLED=1 go test -run '^$' -bench . ./internal/handlers
+}
+
+main "$@"