Rewrite nano::store iterators to use variants instead of polymorphism. #1017

Job	Run time
Linux [UBSAN] [lmdb \| clang]	40m 24s
macOS [UBSAN] [lmdb \| clang]	36m 25s
macOS [ASAN] [lmdb \| clang]	30m 40s
Linux [ASAN] [lmdb \| clang]	42m 39s
macOS [TSAN] [lmdb \| clang]	33m 10s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	41m 35s
macOS [UBSAN] [rocksdb \| clang]	1h 0m 13s
Linux [TSAN] [lmdb \| clang]	44m 18s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	40m 17s
macOS [ASAN] [rocksdb \| clang]	42m 27s
Linux [UBSAN] [rocksdb \| clang]	53m 11s
macOS [TSAN] [rocksdb \| clang]	55m 2s
Linux [ASAN] [rocksdb \| clang]	55m 29s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	58m 19s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	55m 35s
	11h 29m 44s

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