2.6.2 - Fix JsonShopConfiguration saves

[SparklingComet]

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Changes

Fix for bug reported on discord about shop deletion not being saved/reflected in save files. Also introduces a thread (task) pool system for better performance. The previous async saving implementation relying on HashMaps as semaphores/locks was only a band-aid fix.

I have done some basic testing with 1-2 threads, more stress-testing (more queued save jobs, more threads) would be helpful.