Make background thread stoppable

Hi,

Current version of LevelDB has detached background thread that can't be stopped and each environment calls `std::abort()` in destructor.
This behavior prevents using of LevelDB in plugin dlls that can be started and stopped when main application still runs.

I think it is better to allow background thread to finish tasks in queue and then exit, when environment is being destroyed. That will make database loadable and unloadable.