Add submodule lmdbxx which is a c++ wrapper for lmdb #2235

Job	Run time
Linux [UBSAN] [lmdb \| clang]	40m 21s
macOS [UBSAN] [lmdb \| clang]	39m 44s
macOS [ASAN] [lmdb \| clang]	31m 18s
macOS [TSAN] [lmdb \| clang]	32m 53s
Linux [ASAN] [lmdb \| clang]	39m 45s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	43m 31s
macOS [UBSAN] [rocksdb \| clang]	33m 3s
Linux [TSAN] [lmdb \| clang]	39m 58s
macOS [ASAN] [rocksdb \| clang]	39m 50s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	42m 44s
Linux [UBSAN] [rocksdb \| clang]	51m 23s
Linux [ASAN] [rocksdb \| clang]	1h 8m 1s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	57m 15s
macOS [TSAN] [rocksdb \| clang]	55m 27s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	58m 29s
	11h 13m 42s

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