Implement non-blocking thread safety in injector and dependency solver

[ghost]

We can use graph cloning to implement the dependency solver safety, with an atomic reference and compareAndSet().

Similar procedures can be used to do safe inserts into the provider cache. MemoizingProvider is already thread safe, and walking a graph is safe because any new nodes won't affect something in the process of being instantiated.

*Originally reported by mludwig on BitBucket