Add pull through cache functionality to the Registry which can be configured

with a new `proxy` section in the configuration file.

Create a new registry type which delegates storage to a proxyBlobStore
and proxyManifestStore.  These stores will pull through data if not present
locally.  proxyBlobStore takes care not to write duplicate data to disk.

Add a scheduler to cleanup expired content. The scheduler runs as a background
goroutine.  When a blob or manifest is pulled through from the remote registry,
an entry is added to the scheduler with a TTL.  When the TTL expires the
scheduler calls a pre-specified function to remove the fetched resource.

Add token authentication to the registry middleware.  Get a token at startup
and preload the credential store with the username and password supplied in the
config file.

Allow resumable digest functionality to be disabled at runtime and disable
it when the registry is a pull through cache.

Signed-off-by: Richard Scothern <richard.scothern@gmail.com>

docs/storage/blobwriter_resumable.go

@@ -24,6 +24,10 @@ import (
 // offset. Any unhashed bytes remaining less than the given offset are hashed
 // from the content uploaded so far.
 func (bw *blobWriter) resumeDigestAt(ctx context.Context, offset int64) error {
+	if !bw.resumableDigestEnabled {
+		return errResumableDigestNotAvailable
+	}
+
 	if offset < 0 {
 		return fmt.Errorf("cannot resume hash at negative offset: %d", offset)
 	}
@@ -143,6 +147,10 @@ func (bw *blobWriter) getStoredHashStates(ctx context.Context) ([]hashStateEntry
 }
 
 func (bw *blobWriter) storeHashState(ctx context.Context) error {
+	if !bw.resumableDigestEnabled {
+		return errResumableDigestNotAvailable
+	}
+
 	h, ok := bw.digester.Hash().(resumable.Hash)
 	if !ok {
 		return errResumableDigestNotAvailable