Make Cache return Future[T]

[wsargent]

The cache interface is synchronous:

package play.api.libs.ws.ahc.cache

trait Cache {
  def get(key: EffectiveURIKey): ResponseEntry
  def put(key: EffectiveURIKey, entry: ResponseEntry): Unit
  def remove(key: EffectiveURIKey): Unit
  def close(): Unit
}

Should be made to return Future[ResponseEntry] so that IO blocking / disk based cache system will work with it.