Sorting by `INTERNALDATE` may result in lost messages

[link2xt]

Here prefetch returns messages by INTERNALDATE:

core/src/imap.rs

Lines 605 to 608 in 8df9b9e

	let msgs = session
	.prefetch(old_uid_next, uids_to_prefetch)
	.await
	.context("prefetch")?;

Because of this UIDs in this loop come in arbitrary order:

core/src/imap.rs

Line 623 in 8df9b9e

for (uid, ref fetch_response) in msgs {

But here we update largest_uid_fetched assuming UIDs don't decrease:

core/src/imap.rs

Line 754 in 8df9b9e

largest_uid_fetched = max(largest_uid_fetched, uid);

So if e.g. we prefetch UIDs 1, 3, 2, then fetch 1 and 3 but fail before 2, largest fetched UID would be 3 and we will next time not fetch UID 2.

This INTERNALDATE sorting comes from #3789, it is needed to revert the problem of providers that reorder messages when MOVE is used on a batch of messages.

After removing mvbox_move we don't need to sort prefetch by INTERNALDATE and can sort it by UID.