Improve metadata update speed

[ligi]

ideally it should happen as soon as the tune changes.

[olelond]

true - but as long as we're doing poor mans polling from the client i dont think we should fetch more often - we're running 5 sec intervals already

but the polling model was temporary anyway - we need to get some websockets going - but lets wait to see what calendar solution we can come up with since that might change the setup anyway

but we could start by investigating what liquidsoap can do in itself - a simple solution is to have a websocket server running thas does fast polling server side and push'es changes to clients