(Experimental) Pickle: Consider Series identity when unpickling

[franzpoeschel]

This tries fixing the bug reported in #1919 by @pordyna by storing and reading additionally the immutable internal Series memory location. If the location of an unpickled Series is identical to the memory location of a previously unpickled Series, then that Series is reused.

Notes:

• The cache of unpickled Series objects is still thread_local at the moment. This keeps thread safety at the cost of restricting this bugfix to uses in the same thread.
  Suggestion: When unpickling from the same Series object, users should expect to share the same internal data references. So it might be fine to use a per-process cache instead of a per-thread cache.
• The cache is never emptied, so this can be a memory leak. We could maybe add a scan operation that erases closed Series objects from the cache?
• Since our pickling strategy is relatively basic, we will probably keep running into issues like this, as use cases get more complex.

• tqdm as test dependency
• check if we can automatically manage memory, this currently relies on Series::close()

[pordyna]

Do you add tqdm as a requirement for your CI? Otherwise, this will never be tested. You could maybe also rewrite it with Pool().map() from multiprocessing if you want to avoid the extra dependency.

[franzpoeschel]

Not yet, I have added it as a TODO
I want to give the CI one try on the test without fix anyway, so it will come up latest at that point

[ax3l]

pytest has @skipif that we can use for missing imports.

[ax3l]

I agree we can probably reproduce this with multiprocessing from the Python stdlib

[ax3l]

An idea:
Usually when we create series in Python, the Python interpreter owns the lifetime. Now after pickle, there is a C++ object owning the lifetime.

There might be a way to attach the extra data/counter/id to the Python object dynamically and reference it (or the series that is de-serialized first) between each other in Python.