race in serve_rendered.js together with serveAllStyles

[xabbu42]

There is a small race in serve_rendered.js as it returns a promise accessing the passed repo hash. But when the code is actually executed the the key in repo could already be removed again. I managed to trigger this in branch of mine which seemed to slow down stuff at startup enough that this together with the way serveAllStyles handles change events (by removing and readding styles) let the server crash.