improve: use uid preload for stable instanceIDs

[lognaturel]

Closes #228

Warning: I have not tried this.

@issa-tseng Our conversation about instanceID in the context of Central made me think of this. As you can see, this explains why servers don't have to think too much about instanceID collisions -- the IDs are not actually stable across form edit sessions.

As there's talk of better supporting edit workflows and links between forms, stable instanceIDs become important. This change will happen in pyxform soon and since there's a Build release pending, I thought I'd try to squeeze this in at the 23:59th hour...

Feel free to do your own commit if you don't like my commit message or if it's wrong in some way. Or if you'd rather not squeeze something like this in, I understand.

[issa-tseng]

well, if you are sure the generated xml does the correct thing i'm very happy to merge this.

[lognaturel]

For posterity, I verified change-description.xml.txt in Collect 1.22 and confirmed the instanceID does not change across edit sessions. Hopefully you can confirm that the change I made results in XML like that. That is:

<bind jr:preload="uid" nodeset="/change-description/meta/instanceID" readonly="true()" type="string"/>

There's some small amount of risk that somehow users are relying on unstable instanceIDs or that users of old clients will be surprised by the format of the UUID but I think that it's a change we have to make sooner than later.