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.
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