Update to LevelDB 1.12

We're currently on 1.9

Most of the bugs are not consequential. One of them fixes file descriptor exhaustion which I think we already worked around with our own settings overrides. Another fixes an issue where calling CompactRange() manually could result in data being undeleted, which would obviously be a critical security issue if we ever did that. So better to upgrade than accidentally introduce a hole in future by having someone add a CompactRange() call.