Code Flushing: Remove stale code from all theards

Follow up from #705

"Derek Bruening's [PhD thesis](http://www.burningcutlery.com/derek/docs/phd.pdf) describes a system for invalidating code blocks (called "fragments") across multiple thread. The relevant part starts on page 156.

However it is quite complex: you need to be able to hotpatch branch instructions and update lookup tables while another thread is executing translated code. At that point it might be worth simply switching to a single shared code cache, which has many other benefits. See [this paper](http://www.burningcutlery.com/derek/docs/threadshared-CGO06.pdf) for details on implementing a thread-shared code cache." (via @Amanieu)