This patch adds the `OldMachine::mark_all_tracked_users_as_dirty`.

This patch rewrites a bit `OlmMachine::new_helper` by extracting some
piece of it inside `OlmMachine::new_helper_prelude`. With that, we
can rewrite `OlmMachine::migration_post_verified_latch_support` to use
`IdentityManager::mark_all_tracked_users_as_dirty`.
This latter is the shared implementation with
`OlmMachine::mark_all_tracked_users_as_dirty`.

This patch adds a test for `OlmMachine:mark_all_tracked_users_as_dirty`.

There is a non-negligible difference MSC3575 and MSC4186 in how the
`e2ee` extension works. When the client sends a request with no `pos`:

* MSC3575 returns all device lists updates since the last request
  from the device that asked for device lists (this works similarly to
  to-device message handling),

* MSC4186 returns no device lists updates, as it only returns changes
  since the provided `pos` (which is `null` in this case); this is in
  line with sync v2.

Therefore, with MSC4186, the device list cache must be marked as to be
re-downloaded if the `since` token is `None`, otherwise it's easy to
miss device lists updates that happened between the previous request and
the new “initial” request.

…ices