Store a source the way a variant is stored, one row and one file per source URL (closes #308)

A source is now one source_content row and one file named by its
SourceKey, the SHA-256 of its host, path and query, made as the variant
key is. A store writes the file, then upserts the row, and removes the
file when the row cannot be written; an eviction deletes the row, then
the file, with no lock. source_metadata, the JSON files under
cache/metadata/, MetadataStorage, the content lock, the eviction
transaction and its test hook are gone. Two source URLs serving the same
bytes store them twice. Reconciliation still drops a source row whose
file is missing, as TestReconciliationReadsAPageAtATime counts those
reads. The tests of the removed sharing, lock and hook go with them.

Model: opus-5-5

internal/imgcache/partial_cache_files_internal_test.go

@@ -316,10 +316,10 @@ func TestVariantStorage_StoreReplacesMetaFile(t *testing.T) {
 	}
 }
 
-// TestContentStorage_StoreHashedReplacesFileOfAnotherSize checks that a
+// TestContentStorage_StoreReplacesFileOfAnotherSize checks that a
 // store over a file of another size at the content's path, such as a
 // partial file a crash left, replaces it with the content.
-func TestContentStorage_StoreHashedReplacesFileOfAnotherSize(t *testing.T) {
+func TestContentStorage_StoreReplacesFileOfAnotherSize(t *testing.T) {
 	t.Parallel()
 
 	storage, err := NewContentStorage(t.TempDir())
@@ -329,7 +329,7 @@ func TestContentStorage_StoreHashedReplacesFileOfAnotherSize(t *testing.T) {
 
 	content := []byte("the whole source image")
 	sum := sha256.Sum256(content)
-	hash := ContentHash(hex.EncodeToString(sum[:]))
+	hash := imagerequest.SourceKey(hex.EncodeToString(sum[:]))
 	path := storage.hashToPath(hash)
 
 	err = os.MkdirAll(filepath.Dir(path), StorageDirPerm)
@@ -342,9 +342,9 @@ func TestContentStorage_StoreHashedReplacesFileOfAnotherSize(t *testing.T) {
 		t.Fatalf("writing a partial file: %v", err)
 	}
 
-	_, err = storage.StoreHashed(hash, content)
+	_, err = storage.Store(hash, content)
 	if err != nil {
-		t.Fatalf("StoreHashed() error = %v", err)
+		t.Fatalf("Store() error = %v", err)
 	}
 
 	stored, err := storage.Load(hash)