[FR] Dynamic minimum stake

[gztensor]

This request is coming from Rhef. We need a way to better incentivize staking into smaller delegate keys. A major improvement may be achieved with making the minimum stake dynamic based on the number of nominators and/or total stake of the delegate key.

Another way to improve this is introducing a tx fee for staking (and possibly increasing it) for the delegate keys that already have a lot of nominators and/or stake.

TODO: Research this.

[camfairchild]

Currently implementing this for RAO as a top-k stakers limit