Add mutex to protect node.staticChild

[bastiankoetsier]

This PR adds another mutex to protect staticChild for concurrent access.
Additionally, I cleaned up an unnecessary else branch

[dimfeld]

I'm reluctant to put a mutex in the read path, as the vast majority of users aren't adding routes while the router is in use.

Let's use an RWMutex instead, and have just one over the entire tree, probably stored in the router, instead of one per node. And have an option in the router to explicitly enable use of this mutex in the read path. This is so that the atomic operations inside the RWMutex don't slow things down under high load if the user doesn't need the mutex behavior.

[bastiankoetsier]

I understand your point. Maybe when I find the time I can come up with an according solution. Thanks for considering anyway!

[dimfeld]

If I find the time I'll be glad to implement this myself :) It may take a few weeks before I'm able to get to it though.

[dimfeld]

@bastiankoetsier I had time today to do this! Check out #43 and the latest release 3.7.0 to get the new feature.

[bastiankoetsier]

@dimfeld nice! I really appreciate the work you put into this! Thank you very much! Keep on working hard and have a very happy new year!