Sort only Edge fid so edge_id becomes more stable

[visr]

Before we sorted the Edge table like this:

        sort_keys = [
            "from_node_type",
            "from_node_id",
            "to_node_type",
            "to_node_id",
        ]

This made it appear a bit more neat, though it served no other purpose.

The fid index was mostly an implementation detail that the user did not specify, and it went from 1:n following the sorting above. However fid becomes edge_id in the flow.arrow. Therefore when a users added a new edge, usually half of all edge_ids changed, making post-processing unnecessarily difficult. Therefore this PR removes this sorting, such that the input order is retained, keeping the old edge_ids stable.

With this I think we can close #1310. It is useful to have a single identifier value for an Edge, even though it is somewhat superfluous. With this PR it becomes stable, unless users start modifying fids themselves.