[Tilesathome] Key types - single string key instead of multiple numeric keys?

[OJW]

So far, the best suggestion that I got for significantly improving the upload 
speed is to use a single key (i.e. text "x-y-z-layer") instead of 4 
orthogonal fields.  

In preparation for this, I'm adding suitable fields to the database, which 
will be visible as a performance hit.  

Next thing will be to change the PHP code, so if anyone would like to help by 
doing the relevant changes and checking them into SVN, it would help a lot.