`bulk_load_from_existing` doesn't set `min_key_` and `max_key_` correctly

[stoianmihail]

Hi!

There is still a problem that I didn't see when we fixed min_key_ last time.
The function bulk_load_from_existing has left and right which specify the range of the old data node to be considered. Therefore, it is incorrect to say, for example, max_key_ = node->max_key_.

I've prepared a PR.

Thanks!
Mihail.

[jialinding]

Thanks for the catch!