work_pool: move stopping from destructor to stop function #1261

Job	Run time
macOS [UBSAN] [lmdb \| clang]	21m 24s
macOS [ASAN] [lmdb \| clang]	25m 8s
macOS [TSAN] [lmdb \| clang] (Errors Ignored)	26m 53s
macOS [UBSAN] [rocksdb \| clang]	35m 1s
Linux [UBSAN] [lmdb \| clang]	35m 53s
macOS [ASAN] [rocksdb \| clang]	34m 47s
Linux [ASAN] [lmdb \| clang]	36m 25s
macOS [TSAN] [rocksdb \| clang] (Errors Ignored)	43m 12s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	37m 39s
Linux [TSAN] [lmdb \| clang] (Errors Ignored)	35m 59s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	36m 7s
Linux [UBSAN] [rocksdb \| clang]	43m 1s
Linux [ASAN] [rocksdb \| clang]	46m 52s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	47m 31s
Linux [TSAN] [rocksdb \| clang] (Errors Ignored)	1h 32m 38s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	48m 53s
	10h 47m 23s

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