experiments: add state cleanup for deleted experiments

[fsagbuya]

ARTIQ Pull Request

Description of Changes

Implement Least Recently Used (LRU) based management for experiment state cleanup. Use dictionary size limit of 100 as default (can be modified).

NOTE: Requires merging #2590 first.

Related Issue

Closes #2576

Type of Changes

	Type
✓	✨ New feature

Steps (Choose relevant, delete irrelevant before submitting)

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[sbourdeauducq]

Why would you delete the state of an experiment if it's open?

[sbourdeauducq]

And this code is likely never called? See:

    def restore_state(self, state):
        if self.open_experiments:
            raise NotImplementedError
   ...

[sbourdeauducq]

The logic of this seems broken at several levels. Experiments open outside the repos are never cleared. The fact that the cleanup only happens in restore_state doesn't make sense.

Limiting the size of the relevant dictionaries by removing out old entries when a certain size is reached (see https://www.google.com/search?q=python+lru+dict) sounds simpler and more robust.