Fix incorrect addition of parents to ready queue in TwitterAggregateManager.

We need to check parents dirty flags because by the time we're
adding them to the ready queue, we may no longer be their child,
meaning they could have already been processed and no longer be
dirty.
Fix an assertion which is no longer correct in HostedObject.

Because loc update requests now sometimes require async lookups of
query data, they can get coalesced and a second request to send an
update will find no updates that need to be sent out. Previously this
would indicate something went wrong, but now we should just check for
the condition and bail out if there's no work to do.