Segfault with concurrent insertions/deletions using the rbtree

[grtheod]

Hi and thanks for the cool project!

I was playing with the rbtree implementation and while the current benchmarks work as expected, when I tried to have multiple threads performing insertions and deletions at the same time I got a null pointer exception in the removal method.

I was wondering whether this is a known issue and if you plan to address it. :)

Thanks, George