feat: Assign message to ad-hoc group with matching name and members (#5385)

[iequidoo]

This should fix ad-hoc groups splitting when messages are fetched out of order from different
folders or otherwise reordered, or some messages are missing so that the messages reference chain is
broken, or a member was removed from the thread and readded later, etc. Even if this way two
different threads are merged, it looks acceptable, having many threads with the same name/subject
and members isn't a common use case.

Fix #5385

[iequidoo]

Btw, i just received several mails from github and that's how it looks curently: These are unrelated threads, but even Gmail Android app merges them to a signle thread. Once this PR is merged, Delta Chat will do the same.

EDIT: I checked, Gmail app does the same as this PR apparently, it merges threads with the same Subject and participants.