LevelDB (Feature request): Add --leveldb option to enable/disable the engine

Usually for extra engines we have <engine> (and the corresponding skip-<engine>) options to control their usage upon server startup. There is no such option for LevelDB, so if one attempts to use it, LevelDB takes it for a prefix of leveldb-lock-wait-timeout, fails to parse it because it doesn't have a value, and aborts on startup.