Timed execution in eval

[castironpi at gmail.com]

> Write the docs:
>
> > when two threads simultaneously increment the reference count of the same object
>
> Well, the example sucked.  Just synchronize ref count manipulation.
> Every OS has locking primitives, and a library exists to deny requests
> to block that lock dead.  How integral is the GIL to Python?
>
> > The Python interpreter is not fully thread safe
>
> Make it so.

Per-thread reference counts do the trick.