Move memory code out, add it as layer on top of existing donut code

Just like how http.Handlers can be overlayed on top of each other
with each implementing ServeHTTP().

drivers.Driver can be overlayed on top of each other in similar manner
which would implement the drivers.Driver interface.

   API <----> cache <----> donut <----> donut(format)

commands.go

@@ -135,7 +135,7 @@ func runDonut(c *cli.Context) {
 		Fatalln("Path must be set")
 	}
 	apiServerConfig := getAPIServerConfig(c)
-	donutDriver := server.DonutFactory{
+	donutDriver := server.Factory{
 		Config:     apiServerConfig,
 		Paths:      paths,
 		MaxMemory:  maxMemory,