content/local: consistent handling of data and locks

The locks now retry on the backend side to prevent clients from having
to round trip on locks that might be momentarily held. This exposed some
timing errors in the updated_at fields for content ingest, so we've had
to move that to a separate file to export the monotonic go runtime
timestamps.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

content/local/writer.go

@@ -152,6 +152,7 @@ func (w *writer) Close() (err error) {
 	if w.fp != nil {
 		w.fp.Sync()
 		err = w.fp.Close()
+		writeTimestampFile(filepath.Join(w.path, "updatedat"), w.updatedAt)
 		w.fp = nil
 		unlock(w.ref)
 		return