libtransport: store seen message IDs to prevent duplication

- Message IDs get stored in a database now (in all cases, not just
  groupchats), and we won't resend a message with the same ID as
  a previous one.
- Because stuff gets hashed, this should provide reasonable
  protections against backends/plugins not doing the whole read state
  thing.
- Also, we've just stopped supporting sqlite entirely for now.