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/doc.go

@@ -1,3 +1,3 @@
 // Package registry is a placeholder package for registry interface
-// destinations and utilities.
+// definitions and utilities.
 package registry