Make link roads the same z_order as their base class

[pnorman]

By adding link status to the ORDER BY this then allows the ordering
logic to be changed at low zoom or in forks.

Tested locally at several complex interchanges and everything looks as it does on tile.osm.org.

Fixes #3719

[jeisenbe]

This reminds me that we had intended to add construction=* to the z_order list to fix inconsistent ordering of highway=construction types, as mentioned in #3579 and the initial version of PR #3646.

Would you be willing to add the z_order for different values of construction= to this PR, for example something like this commit: 810448b ? Or else I can open a separate PR.

[pnorman]

Let's keep them in different PRs

[jeisenbe]

I checked test renderings with various types of link roads, paths, tracks, service, residential etc and it is unchanged. - Oops, I checked out the wrong branch, let's try that again.

[jeisenbe]

Actually, now that I've checked out the right branch, there are some visual changes in rare situations, when a highway=path, track, service or similar low-priority road crosses or meets a link.

These changes are not huge, but perhaps they were not intentional?

Before:

• Links above service/road/path/track

After:

• Links below

Can we fix this?

[pnorman]

Thinking about this, it's not going to work with our peculiar ordering of links between high-class roads and low-class roads.