Core: ThreadPool improvements #1842

Job	Run time
Linux -> Pre-checks	28s
Linux -> Clang-x64, Release	24m 23s
Linux -> GCC-x64, Release	19m 10s
Linux -> Clang-x64, Debug	22m 22s
Linux -> Clang-ASAN-x64, RelWithDebInfo	27m 0s
Linux -> GCC-x64, Debug	22m 17s
Linux -> Clang-TSAN-x64, RelWithDebInfo	29m 57s
	2h 25m 37s

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