We need to be able to expire message store cache keys.

Currently they're mostly needed for the UI for paging, at some point the set becomes too large to page and so storing all those keys is useless & operationally costly.

This ticket is meant to list all places where all message store keys are retrieved from the cache and find ways of not having to cache all keys all the time:

- [x] counts of messages sent & received
- [x] download of all messages in a time ordered fashion.