Add submodule lmdbxx which is a c++ wrapper for lmdb #2228

Job	Run time
Linux [UBSAN] [lmdb \| clang]	15m 9s
macOS [UBSAN] [lmdb \| clang]	27m 44s
Linux [ASAN] [lmdb \| clang]	14m 49s
macOS [ASAN] [lmdb \| clang]	30m 15s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	14m 43s
macOS [TSAN] [lmdb \| clang]	31m 22s
Linux [TSAN] [lmdb \| clang]	14m 33s
macOS [UBSAN] [rocksdb \| clang]	29m 34s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	14m 55s
macOS [ASAN] [rocksdb \| clang]	39m 52s
Linux [UBSAN] [rocksdb \| clang]	14m 40s
macOS [TSAN] [rocksdb \| clang]	58m 33s
Linux [ASAN] [rocksdb \| clang]	14m 47s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	14m 54s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	15m 45s
	5h 51m 35s

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