On month change, do not hide previous layer until tiles are loaded

[bartromgens]

When switching month, the map becomes white/empty until the new month is loaded, this does not help with the smoothness of the experience.

It may help to not clear the previous month image layer until the new layer is loaded. The new layer can have a higher z-index to make sure new tiles are shown on top of the old tiles.

After all tiles are loaded, the previous layer can be hidden.

Example code that may help with this:
http://openlayers.org/en/master/examples/tile-load-events.html