Convert store iterators to variants rather than using polymorphism. #2357

Job	Run time
Linux [UBSAN] [lmdb \| clang]	40m 22s
Linux [ASAN] [lmdb \| clang]	40m 36s
macOS [UBSAN] [lmdb \| clang]	38m 10s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	41m 2s
macOS [ASAN] [lmdb \| clang]	30m 42s
Linux [TSAN] [lmdb \| clang]	40m 31s
macOS [TSAN] [lmdb \| clang]	33m 48s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	42m 55s
macOS [UBSAN] [rocksdb \| clang]	39m 38s
Linux [UBSAN] [rocksdb \| clang]	52m 0s
macOS [ASAN] [rocksdb \| clang]	44m 18s
Linux [ASAN] [rocksdb \| clang]	55m 43s
macOS [TSAN] [rocksdb \| clang]	55m 32s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	56m 16s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	56m 2s
	11h 7m 35s

Provide feedback