Integrate economic model into StakingRewardsProvider trait implementation

[shannonwells]

Now that an economic model (EM) has more or less been decided, use the values and caps described in the EM doc to calculate reward pool size and individual staking rewards.

Acceptance criteria:

1. update reward_pool_size and staking_reward_total to return correct values as described in the EM doc by integrating the values and caps in the document.
2. Tests that prove this using the examples in the document as well as any other needed test cases that exercise the model's limits
3. Change the trait and implementation of StakingRewardsProvider: remove 'from_eraparameter fromstaking_reward_total`, since we should always pay out the oldest staking rewards.
4. Tests for same

Possibly: Update/create (it's currently in a branch and may be out of date enough to start over) the Design Doc for the staking rewards economic model

Related/includes:

• [x] #1569
• [ ] #1571