Allow single threaded use of Python

[cnuernber]

Not all python libraries tolerate multithreaded access well, even when serialized by a GIL. Some, for instance, use static/thread-local variables in a way as to make then unsafe to use from multiple threads.

We want to make purely single threaded access possible when necessary for robustness. It should be possible to load python and use it from an agent and guarantee all python access is from one thread.

One potential soution is a custom gc release-refcount queue that is cleared at the beginning or end of with-gil.