leveldb properties option bug fix

[min-guk]

Existing Leveldb properties options had a "Integer underflow" bug on default values.

size_t cache_size = std::stol(props.GetProperty(PROP_CACHE_SIZE, PROP_CACHE_SIZE_DEFAULT))

When we use default option of cache, r-value is long(-1).
But type size_t doesn't support negative value, therefore "Integer underflow" occurs.

Thanks.

[ls4154]

Thanks for your PR! :)
I've deleted leveldb.properties from the commit since it is unrelated to this patch.