Allow tile cache key calculation to be overridden
Store revision number in sources and update it on changes
Don't use rendered framebuffer when source revision has changed
Handle tile source revisions in DOM tile layer renderer
Add getParams/updateParams to TiledWMS
Add getDimensions/updateDimensions to WMTS
More robust coordinate keys for WMTS and TiledWMS

This uses more robust ZXY coordinate keys, and caches the key to avoid creating garbage each time getKeyZXY is called.