Avoid iterating all operator during epoch transition

[NingLin-P]

During epoch transition, all operators are iterated over, even though we have short-circuit optimization to skip further processing for operators who have no deposit/withdraw/reward within this epoch, we still need to read the operator state first to determine if we can safely skip such operator.

Thus currently a database read is required for every operator during the epoch transition and because the total number of operators of a specific domain is unbounded, the weight of the epoch transition is also unbounded.

One way to fix this is to manually track operators who have deposit/withdraw/reward happened within the current epoch, and during epoch transition we only need to process these operators and leave the rest of the operators untouched.