Refuse splicing the live timeline into a broken position
Credit to Matthew for basically solving this.
Theoretically fixes spontaneous timeline corruption: https://github.com/vector-im/riot-web/issues/8593
When the live timeline ends up in a position where it can no longer be live (such as becoming the second timeline in the set, rather than the first) we end up getting neighbouring timeline errors. By refusing to splice the live timeline into such a position, we hopefully keep the live timeline in a position of still being live for when it is next used.
The running theory that leads to this fix is multiple limited syncs coming in, causing holes in the timeline. When trying to patch up the holes, the timeline set would end up splicing all over the place, leading to potentially splicing the live timeline into a broken position.
