Convert store iterators to variants rather than using polymorphism. #2355

Job	Run time
Linux [UBSAN] [lmdb \| clang]	40m 17s
macOS [UBSAN] [lmdb \| clang]	34m 8s
macOS [ASAN] [lmdb \| clang]	30m 52s
macOS [TSAN] [lmdb \| clang]	33m 53s
macOS [UBSAN] [rocksdb \| clang]	42m 15s
Linux [ASAN] [lmdb \| clang]	40m 9s
macOS [ASAN] [rocksdb \| clang]	39m 16s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	41m 18s
macOS [TSAN] [rocksdb \| clang]	53m 16s
Linux [TSAN] [lmdb \| clang]	43m 38s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	42m 47s
Linux [UBSAN] [rocksdb \| clang]	51m 53s
Linux [ASAN] [rocksdb \| clang]	57m 13s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	57m 22s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	58m 42s
	11h 6m 59s

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