Display WA backlog messages received while bridge was offline

- We now store each WhatsApp message ID we've received in the database,
  so we can determine whether an incoming message has already been
  delivered or not.
- 'Old' messages (those without the 'new' flag) are now delivered,
  instead of merely being thrown away, with a timestamp prepended to
  indicate that they're part of the backlog.
- A new config option `backlog_start` under the [whatsapp] section
  enables users to hide backlog messages before a given timestamp, specified
  in ISO 8601 format. This is useful if you're migrating to this version
  and don't want a million copies of all your messages to be sent.
- The `ignore_other_libraries` config flag was fixed to actually work.