Rework `nano::thread_pool` #2339

Job	Run time
macOS [UBSAN] [lmdb \| clang]	40m 52s
Linux [UBSAN] [lmdb \| clang]	44m 46s
macOS [ASAN] [lmdb \| clang]	34m 6s
Linux [ASAN] [lmdb \| clang]	46m 31s
macOS [TSAN] [lmdb \| clang]	32m 1s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	41m 46s
macOS [UBSAN] [rocksdb \| clang]	45m 18s
Linux [TSAN] [lmdb \| clang]	45m 43s
macOS [ASAN] [rocksdb \| clang]	43m 45s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	47m 40s
macOS [TSAN] [rocksdb \| clang]	1h 5m 30s
Linux [UBSAN] [rocksdb \| clang]	1h 0m 22s
Linux [ASAN] [rocksdb \| clang]	55m 21s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	56m 40s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	1h 0m 0s
	12h 0m 21s

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