Sequential and cancelable stats reloads

[supersaiyansubtlety]

Fixes #271

Makes it so that Gui::reloadStats:

• attempts to cancel any incomplete asynchronous work it started in previous calls
• waits for any un-cancelable prior work to complete before starting the current call's work

I also made ProjectStatsResult.stats a ConcurrentHashMap because I got this ConcurrentModificationException once:
enigma_crash-concurrent-reloadStats.log.

I think the exception was possible but rare before this PR, but it was inconsistent so I can't be certain.

Supersedes #286

[ix0rai]

this looks great! you can definitely feel the improvement, especially when first loading the project. i'm confident this solves the issue in the backend, but it's still possible for the UI to go from an incorrect icon to another incorrect icon before landing on the correct one, thanks to the extremely slow reloads on the jtree rendering. honestly, i'm kinda wondering how much effort it would be to try to make a faster jtree with better management for what's actually loaded