gh-124218: Use per-thread refcounts for code objects (#125216)

gh-124218: Use per-thread refcounts for code objects (#125216)

Use per-thread refcounting for the reference from function objects to
their corresponding code object. This can be a source of contention when
frequently creating nested functions. Deferred refcounting alone isn't a
great fit here because these references are on the heap and may be
modified by other libraries.