Track memory in C-only threads as belonging to Python code launched them

[itamarst]

BLAS (via NumPy), Blosc (via Zarr), etc. will launch short-run threads. They only run Python code. Right now Fil sets env variables to not launch threads. It would be better to assign their allocations to Python code that launched them—a good heuristic is "thread that is only C code gets assigned to Python code that launched it".

Benefits:

1. Fil will run code the same as regular code; right now it tries to disable threading for things it knows about.
2. It will track memory for things Fil doesn't know about, including custom C code.

[itamarst]

Implementation idea: new threads get launched with default callstack ID of the code that launched them. Python thread initialization resets the callstack.