Removing mailbox index broke seen flag synchronization etc.

[link2xt]

#632 broke some core tests.

tests/test_something.py::test_markseen_contact_request is now waiting forever because message marked as seen on one device is not causing IMAP notification on another device.

Created folder is not seen on another IMAP connection anymore until IMAP connection is restored: chatmail/core#7206

It seems index file was used to share such updates, one IMAP connection changes flags and another connection detects this via index update, but now each IMAP connection has its own index.

[link2xt]

For Seen flag synchronization core can workaround this by sending MDNs to self probably: chatmail/core#7005

[missytake]

Should we revert #632 for now, until seen flags are synchronized via MDNs?

[link2xt]

I opened #643