Debug multitransport

[link2xt]

This is a WIP PR on top of #7348, somehow it makes CI get stuck and timeout so I have split it off.

EDIT: the bug is that during new transport configuration transport_id 0 is used, so UID validity is set for 0 and then gets reset when reconnected and messages are skipped.

The problem can be reproduced when running on the server, tests randomly fail:

FAILED tests/test_securejoin.py::test_setup_contact_resetup - Failed: Timeout (>60.0s) from pytest-timeout.
FAILED tests/test_something.py::test_import_export_keys - Failed: Timeout (>60.0s) from pytest-timeout.
FAILED tests/test_something.py::test_delete_deltachat_folder - Failed: Timeout (>60.0s) from pytest-timeout.

But on a slower wi-fi network that has higher ping to the email server this never fails for me.

Anyway, need to fix it, then merge back into #7348