[Tilesathome] layer sanity check

[spaetz]

I just addeed some sanity checking to the server, so it's a bit smarter about which layers it will accept.

It will now refuse/drop layers that have requested zoom values that are not configured on the server.
Specifically, it will refuse lowzoom requests for z12 tilesets. I need to go through the db and purge it from those requests manually, but we should see somewhat less client errors because of nonsensical requests.

spaetz