storage: streamline ETag calculation

This commit introduces some helpers so that ETags can be calculated at
the same time that files get read or written. Besides looking nicer, it
should also help reduce lock contention around file access, as files do
not need to be opened twice anymore.

storage/filesystem.go

@@ -1,8 +1,6 @@
 package storage
 
 import (
-	"crypto/sha1"
-	"encoding/base64"
 	"fmt"
 	"io"
 	"os"
@@ -144,11 +142,10 @@ func etagForFile(path string) (string, error) {
 	}
 	defer f.Close()
 
-	h := sha1.New()
-	if _, err := io.Copy(h, f); err != nil {
+	src := NewETagReader(f)
+	if _, err := io.ReadAll(src); err != nil {
 		return "", err
 	}
-	csum := h.Sum(nil)
 
-	return base64.StdEncoding.EncodeToString(csum[:]), nil
+	return src.ETag(), nil
 }