[2.0rc2] RackController.get_image_sync_status causes huge load on regiond process

The call in the handlers/controller.py to get the image sync status causes huge load on the regiond process. The call stack does like this:

ControllerHandler.check_images
node.get_image_sync_status()
BootResource.objects.boot_images_are_in_sync(boot_images) <- This is where it gets slow.

The reason it is slow is the way MAAS calculates the size of the each largefile in the database. It does this by opening each largeobject seeking to the end and getting the size. This was fine in 1.9 when the cluster page was not polled like it is today in MAAS 2.0.

    @property
    def size(self):
        """Size of content."""
        with self.content.open('rb') as stream:
            stream.seek(0, os.SEEK_END)
            size = stream.tell()
        return size

This needs to be converted to a field in the database holding the current size of the largefile instead of calculating in from the largeobject. This will speed up the BootResource.objects.boot_images_are_in_sync method. Also a more advance SQL query could be used to make less calls to check if the images are in sync.

This is currently affecting OIL, to alleviate the issue the following was performed:

class RackController(Controller):

   ...blah...

   def get_image_sync_status(self, boot_images=None):
       return 'unknown'
       ... the original function...