[IronPython] Thread local storage in IronPython 2.6

[Dino Viehland]

> *However*, we discovered that we inadvertently had options["Frames"] =
> true; in the code that creates the main engine. Switching that off (and
> then having to recompile all our Python code) solved the problem.
> 
> It may indicate that if we want to turn frames on in individual engines
> (each recalculation thread has its own Python engine) then we may have a
> problem - but it is no longer blocking us. Sorry for the noise.

Ideally we wouldn't leak memory with frames on though!  :)  In theory
when a new thread gets created we will replace the old threads memory and
everything can be reclaimed.  But I could change this so that we use a 
normal thread static which will get cleaned up eagerly (it'll have some 
small performance impact when frames are enabled though).