Alter reward distribution logic to take account of prior eras' delegator identities and weights

[Sainath Sherigar]

In feat-2.0 rewards are distributed to delegators according to the current (distribution) era block identities and weights, but with lookback some of the rewards have to be distributed in accordance to a prior era's delegator identities and weights (while also taking into account the validator's prior era share of the total stake)