Convert store iterators to variants rather than using polymorphism. #2354

Job	Run time
Linux [UBSAN] [lmdb \| clang]	40m 25s
macOS [UBSAN] [lmdb \| clang]	37m 39s
Linux [ASAN] [lmdb \| clang]	40m 12s
macOS [ASAN] [lmdb \| clang]	29m 28s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	40m 58s
macOS [TSAN] [lmdb \| clang]	31m 41s
Linux [TSAN] [lmdb \| clang]	41m 32s
macOS [UBSAN] [rocksdb \| clang]	41m 17s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	39m 59s
macOS [ASAN] [rocksdb \| clang]	41m 33s
Linux [UBSAN] [rocksdb \| clang]	52m 18s
macOS [TSAN] [rocksdb \| clang]	53m 27s
Linux [ASAN] [rocksdb \| clang]	58m 27s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	57m 23s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	59m 11s
	11h 5m 30s

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