Validator Damping Function

[shawntabrizi]

Context: https://kusama.polkassembly.io/post/250

There is a value in the runtime which determines the number of validators the NPoS election algorithm will pick.

We want to use a damping function to control this number between an upper and lower bound based on the distribution of stake/nominations on the network.

TODO:

• [ ] Sanity check current implementation
• [ ] Improvements if any to make the algorithm more clean/readable
• [ ] Configurable pallet flag to disable or enable this feature
• [ ] Integration Tests
• [ ] Maybe a little bit of modeling to help outside people understand behavior in diff scenarios

[shawntabrizi]

@g2udevelopment will be looking to finish this one up.