Improve hover tooltip cache invalidation

[krassowski]

The tooltip cache introduced in #363 has many advantages, but the cache invalidation is not perfect.

• the cache can get outdated if a file from which something is imported changes; currently we do not have any way to invalidate this. We could schedule a refresh of a cached response to run in the background while we show the most recent response we have in the meantime.
• the cache does not invalidate on undo triggered by ctrl + z. It could be because this might be managed by the lab and not by the CodeMIrror. It would need to listen to the model changes, but model is not exposed to the features at the moment!