Efficiently compute and synchronize used flags

1. Store used flags per-folder instead of computing them each time.
2. Update stored flags on each relevant folder change
3. Avoid updating flags if it is redundant (e.g. new message does not use new flags).
4. Keep track of per-session known used flags and send update on synchronization (Poll, Idle).

Note that standard IMAP flags are always considered to be used, we don't need to keep track of them.