* Include a `bk` column to store the storage backend
  * Include a `signature` column which represents a repeatable hash of the
    file contents
  * Rename `hash` to `key` since it isn't a real hash

This is instead of optimizing the lock table for every cron run, which
wastes a lot of db server io

This includes the initial concept of pluggable authentication along with the
initial concept for a plugin management system.

Also add a cron job to perform the task on demand
Change the implementation for Ticket::deleteAttachments to call this method
after simply removing all entries from the %ticket_attachment table