Implement off-chain Window PoSt verification (#1327)

This patch implements two changes:

1. New Window PoSts, except those that restore faulty sectors, are
   optimistically accepted, assumed correct, and recorded in the
   state-tree for one proving period.
2. Optimistically accepted window posts can be disputed until
   `WPoStProofDisputeWindow` epochs after the challenge window in which
   they were submitted closes. When a dispute successfully refutes an
   optimistically accepted Window PoSt, the miner is fined one IPF per
   active sector in the partition (at the moment when the proof was
   submitted) plus a flat fee of 20FIL, all incorrectly proved sectors
   are marked faulty, and the disputer (address that submitted the
   dispute) is rewarded a fixed `DipsuteReward`.

actors/builtin/methods.go

@@ -99,7 +99,8 @@ var MethodsMiner = struct {
 	ConfirmUpdateWorkerKey   abi.MethodNum
 	RepayDebt                abi.MethodNum
 	ChangeOwnerAddress       abi.MethodNum
-}{MethodConstructor, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23}
+	DisputeWindowedPoSt      abi.MethodNum
+}{MethodConstructor, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24}
 
 var MethodsVerifiedRegistry = struct {
 	Constructor       abi.MethodNum