Cleanly delete statuses when the recipient is deleted

Right now, when a recipient is deleted, we manually delete associated statuses from the database instead of loading them and calling statuses_delete_status(). The reason for doing this was that loading all the statuses and deleting them one by one would be slow. However, this judgment was a premature optimization -- I haven't done any profiling to see whether this is actually a performance concern, and anyway core does fully invoke all of the relevant node deletion hooks when a user is deleted for each individual node (node_delete_multiple() called by node_user_delete()). Deleting a node is a lot more expensive than deleting statuses -- but on the other hand there could be a lot more statuses attached to an entity than nodes. And deleting a status is probably about as fast as queuing it to process later during cron, so I'd be surprised if there was much advantage there.

Solution:
Let's just fully load each status and call statuses_delete_status() on each when we delete a sender or recipient. Using the try-catch model that node_delete_multiple() uses should prevent errors from screwing things up. We should not use transactions here since we are already nested inside the transaction from user_delete_multiple. There is still the threat of timeouts, but let's just hope that no one deletes an object with 500,000 statuses attached to it.