Integrate layer info cache with registry and storage

This changeset integrates the layer info cache with the registry webapp and
storage backend. The main benefit is to cache immutable layer meta data,
reducing backend roundtrips. The cache can be configured to use either redis or
an inmemory cache.

This provides massive performance benefits for HEAD http checks on layer blobs
and manifest verification.

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

registry/storage/filereader.go

@@ -27,8 +27,8 @@ type fileReader struct {
 
 	// identifying fields
 	path    string
-	size    int64 // size is the total layer size, must be set.
-	modtime time.Time
+	size    int64     // size is the total size, must be set.
+	modtime time.Time // TODO(stevvooe): This is not needed anymore.
 
 	// mutable fields
 	rc     io.ReadCloser // remote read closer