startup: Async-ify cached item loading on startup, add timers.

In an attempt to get rid of processEvents() (related to several existing
crash bugs) I am trying to port the startup code towards more
async-friendly schemes.

There's no threading but we at least get back to the event loop much
more frequently while loading files.

Additionally I have added debug output with instrumentation to show how
long it takes to advance through each step of the startup (I think this
might be the first time anyone has understood JuK startup sequence in
years).

This leaves some essentially dead code with Cache (which no longer
acts as a container), which I will try to cleanup in later commits.