Add a blind trash_messages_where allowing the server to select and
delete messages in a single call.  The goal is to make deleting parliament,
attack, or other types of messages easier and faster, with a reduced load
on the server.
Replace the message-update code with direct-to-database updating for
performance reasons.  Also change the return to better match the ideas
behind the trash_messages and archive_messages calls so that the UI can
use the ID list to remove stuff from its display directly, should the UI
ever start using this call.