Proto: optimizing the SWRR selected_bakers

What

Depends on !20180

Implement a cache for SWRR selected_bakers and use FallbackArray instead of List for storage.

Why

We use the cache to reduce the number of calls to the storage, similar to the Alias sampler. We use FallbackArrays to have constant time cost for accesses in the array. That makes the get_baker function constant time.

How

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR