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>

configuration/configuration.go

@@ -128,6 +128,8 @@ type Configuration struct {
 			IdleTimeout time.Duration `yaml:"idletimeout,omitempty"`
 		} `yaml:"pool,omitempty"`
 	} `yaml:"redis,omitempty"`
+
+	Proxy Proxy `yaml:"proxy,omitempty"`
 }
 
 // LogHook is composed of hook Level and Type.
@@ -430,6 +432,18 @@ type Middleware struct {
 	Options Parameters `yaml:"options"`
 }
 
+// Proxy configures the registry as a pull through cache
+type Proxy struct {
+	// RemoteURL is the URL of the remote registry
+	RemoteURL string `yaml:"remoteurl"`
+
+	// Username of the hub user
+	Username string `yaml:"username"`
+
+	// Password of the hub user
+	Password string `yaml:"password"`
+}
+
 // Parse parses an input configuration yaml document into a Configuration struct
 // This should generally be capable of handling old configuration format versions
 //