Improve votings efficiency

[CoderZhi]

Fetching block/epoch/foundation reward is very slow, which takes more than 7 minutes. It may be improved on the iotex-core side, but the way we are fetching it per epoch could get analytics stuck for a long time. https://github.com/iotexproject/iotex-analytics/blob/master/indexprotocol/votings/stakingprotocol.go#L426-L436