Reuse debug symbol groups in CDB for faster performance when switching between stack frames

In the pre-python cdb extension, there was code that took care to re-use the debug symbol groups when possible, making switching between stack frames snappy.
With the new python-cdb extension, that does not seem to be the case, and thus debug symbols are always re-queried when switching between stack frames, which causes the locals view to take very long to update, especially with libraries that have many debug symbols like WebEngineCore.

It would be nice if the new extension would reuse and cache as much as possible, to make the debugging experience more fluid (switching stack frames in Chromium code can take up to 2-5 minutes).