Update computation method for staker rewards

[jtse0]

As mentioned in issue #123 - calculation for staker rewards should take into account the holdings of the user. Formula will be updated to: (staked SLM / total Staked SLM) * reward pool * reward percentage

Each user holding and total holdings will be recorded in the Staker contracts.

[solomondefi-dev]

This will need some slight clarification, it may be necessary to add a constant modifier or other formula adjustment in order to avoid depleting the reward pool too quickly.

[jtse0]

Sorry I actually also updated the formula to include a reward percentage for each payment. We will input a certain reward percentage each payment which will be multiplied by the reward pool each time and then each staker will get their stake of the payment. This reward percentage will just be an extra parameter entered when a new reward payment is announced.