Split away folder and messages from temporary flags

[matzipan]

Currently the messages and folder each have one data structure with all the fields. Fields like uid_validity or fother flags might change, but the programmer might still use the data that is already in the datastructure. This might lead to the developer using stale data.

This task is to remove these ephemeral fields from the structure and move to a separate API that can be used easily to always fetch the latest data.