whatsapp: add support for ack / read receipt tracking

- We now track the MessageAcks received for each WA message we send.
- If we don't receive acks for a given message, the user gets warned
  about this and can configure the bridge to auto-resend the message
  after a given delay.
- ReportFailure now generates a less ignorable PRIVMSG that mentions
  the admin, instead of a NOTICE.
- Log levels for things like contacts being changed or chat actions
  have been toned down somewhat, to avoid spamming the logs.