Implement message archiving by conversation ID.

[IngCr3at1on]

Things to consider:

• Cache but don't archive private messages between guests (if guests are enabled). Messages between guests and registered users should be archived as normal so that the registered users may retrieve them. Cached guest messages should be retained until the guest disconnects that way if they decide to register the conversations can be archived properly for them.
• Archive should only contain conversation data (not participants).
• Implement efficient database pagination using message timestamps (this will never be fetched in real-time).