Add submodule lmdbxx which is a c++ wrapper for lmdb #2229

Job	Run time
Linux [UBSAN] [lmdb \| clang]	39m 41s
macOS [UBSAN] [lmdb \| clang]	23m 0s
Linux [ASAN] [lmdb \| clang]	38m 48s
macOS [ASAN] [lmdb \| clang]	30m 15s
Linux [ASAN_INT] [lmdb \| clang] (Errors Ignored)	39m 59s
macOS [TSAN] [lmdb \| clang]	32m 56s
Linux [TSAN] [lmdb \| clang]	39m 37s
macOS [UBSAN] [rocksdb \| clang]	29m 26s
Linux [LEAK] [lmdb \| clang] (Errors Ignored)	41m 39s
macOS [ASAN] [rocksdb \| clang]	39m 14s
Linux [UBSAN] [rocksdb \| clang]	50m 52s
macOS [TSAN] [rocksdb \| clang]	54m 23s
Linux [ASAN] [rocksdb \| clang]	57m 35s
Linux [ASAN_INT] [rocksdb \| clang] (Errors Ignored)	56m 22s
Linux [LEAK] [rocksdb \| clang] (Errors Ignored)	56m 26s
	10h 30m 13s

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