[3.18.x] A few LMDB related fixes and improvements #939

Job	Run time
shellcheck_tests / Run shellcheck on shell scripts	1m 4s
unit_tests / Run Unit Tests	2m 28s
acceptance_tests / acceptance_tests	20m 23s
macos_unit_tests / macos_unit_tests	7m 4s
valgrind_tests / valgrind_tests	4m 48s
asan_unit_tests / asan_unit_tests	2m 25s
	38m 12s

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