Sorting produces invalid mutation order

[benjeffery]

Adding

tables1.tree_sequence()
tables2.tree_sequence()

at https://github.com/tskit-dev/tskit/blob/main/python/tests/test_tables.py#L1409 causes: _tskit.LibraryError: Parent mutation ID must be < current ID

[petrelharp]

I think the problem isn't so much "produces" but "doesn't fix" (as in #1197). So, arguably, this isn't a bug but an enhancement (unless there's actually some tables that sort breaks). But I agree, it's an enhancement that we should do.

[jeromekelleher]

Adding this to 1.0 for now, but can probably bump if we don't decide to fix the mutation ordering for that.

[benjeffery]

Fixed in #3257