GH-133231: Changes to executor management to support proposed `sys._jit` module ...

GH-133231: Changes to executor management to support proposed `sys._jit` module (GH-133287)

* Track the current executor, not the previous one, on the thread-state.

* Batch executors for deallocation to avoid having to constantly incref executors; this is an ad-hoc form of deferred reference counting.