Improve conversation viewer loading performance
Every time when a big mailing discussion is clicked, such as a gitlab thread with hundreds of comments, Geary downloads it from database anew. This takes considerable amount of time and CPU, and it's hard to work with lots of flashing mails while the process is ongoing.
It would be great if geary have cached threads already viewed in session in RAM (that's the simplest heuristic, but there's definitely a room for further improvements, since the first time a thread clicked, the process would still be noticeable).
Edited by Michael Gratton