resourceloader: Update MessageBlobStore documentation and code cleanup

resourceloader: Update MessageBlobStore documentation and code cleanup

* Update and improve class documentation.

* Remove comment that claims to return false if the module has no message.
  generateMessageBlob() returns '{}' in that case. And this is expected since
  we want to cache the absence of messages.

* Re-use the module objects passed to MessageBlobStore::get() in getFromDB()
  instead of using ResourceLoader::getModule(). The object is the same either
  way (since ResourceLoader re-uses the objects also) but reduces complexity
  of getFromDB() to not need the ResourceLoader object as parameter.

Change-Id: I89a14d7185877fae52791f6837883ed3a6749cd7