Power.OnEpochTickEnd: preempt Miner queries to Power and Reward actors

[wadealexc]

Each time the Power actor invokes Miner.ConfirmSectorProofsValid, the Miner makes several queries:

specs-actors/actors/builtin/miner/miner_actor.go

Lines 613 to 619 in 70a18df

	func (a Actor) ConfirmSectorProofsValid(rt Runtime, params *builtin.ConfirmSectorProofsParams) *adt.EmptyValue {
	rt.ValidateImmediateCallerIs(builtin.StoragePowerActorAddr)
	// get network stats from other actors
	baselinePower, epochReward := requestCurrentEpochBaselinePowerAndReward(rt)
	pwrTotal := requestCurrentTotalPower(rt)
	circulatingSupply := rt.TotalFilCircSupply()

The Power actor should make these queries as part of its OnEpochTickEnd method, and pass them in as additional parameters.