Proto/swrr: initial implementation

What

Implementation of SWRR(Smooth Weighted Round Robin) algorithm to be used for delegate sampler.

Why

We want to remove Alias method.

How

We compute the list of round 0 bakers for a target cycle and keep it in the storage. The algorithm also keeps a persistent state of credits across cycles, which is also kept in the storage.

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