Various safety changes, bugfixes and added null type

[tims-bsquare]

Let me know @ceifa if you'd like this broken down into parts. These are the last of the changes in my private fork and I can appreciate you may not want to upstream all of them.

• Support timing out callbacks from JS into Lua
• Made calling functions from JS into Lua more robust. Previously there was a failure cause where if you created a thread, had that thread return a function, close the thread and attempt to call the function it would try to call the Lua function from within the context of a closed thread. All function calls from JS into Lua now use there own individual calling thread as Lua does when calling C functions and those threads are within a thread only used for functions within the Lua reference registry. This had an interesting side-effect that setInterval blocked the tests from completing because it no longer silently errored.
• Inject null type
• Protect against unable to allocate memory for thread creation
• Fix thread missing parent reference in some cases

[ceifa]

Thanks for the recent changes! @tims-bsquare @Sapu94