Refactor mailbox access to store a cursor in the session

[pheller]

The Messaging table index is presently set in an inflexible and fragile manner, and users paginate through messages with limit/offset. It is preferable to use a more robust auto-increment index for the table, and store a cursor in the Session struct such that pagination is deterministic (and simpler).

• Populate the Messaging data in the Session struct. #47
• Refactor mailbox accessors to use Messaging state in the Session struct #48
• Refactor the Messaging database schema index to simple auto-increment #49

[pheller]

Fixed in #97