Reference cached beacon states with WeakRefs

[dapplion]

Teku does it with good results. Since the cache memory size is technically unbounded this strategy is great for preventing OOMs.

• The head state should be kept separately with a strong reference.
• Test how JS chooses to garbage collect multiple WeakRef. By age? By least used?
• Test performance of WeakRef.unref() if it's called very often (attestation processing)
• Consider adding a shufflings cache to mitigate the risk of a key state getting dropped
• Don't regen for attestation verification if the head state already has the correct shuffling

Note: this is a huge en-devour with uncertain payoff. Teku has had issues in the past with this approach as all states disapears and the node death spirals

• Also see Keep states in cache as WeakRef #3420

[philknows]

Refer to #6008 . May not be required anymore.

[wemeetagain]

@twoeths I don't think this is necessary anymore after n-historical-states?

[twoeths]

yes, block state cache is bounded by max size and checkpoint state cache is bounded by max epoch count