Fix Concurrency exercise solution.

[Ripounet]

The suggested solution, though clear and elegant, has the following flaws :

• it does not handle correctly trees of having different sizes.
• it does not distinguish a zero Value from a non-existant node value.

See my examples, before and after :

• http://play.golang.org/p/rZSOxNjsGe
• http://play.golang.org/p/-tcluDE4rl

However I cannot guarantee right now that there are no goroutine leaks and channels leak (when Same has returned but values are still waiting in ch1 and ch2).