watchtower/wtpolicy: add RewardBase to Policy

watchtower/wtpolicy/policy.go

@@ -48,6 +48,11 @@ type Policy struct {
 	// for this session.
 	MaxUpdates uint16
 
+	// RewardBase is the fixed amount allocated to the tower when the
+	// policy's blob type specifies a reward for the tower. This is taken
+	// before adding the proportional reward.
+	RewardBase uint32
+
 	// RewardRate is the fraction of the total balance of the revoked
 	// commitment that the watchtower is entitled to. This value is
 	// expressed in millionths of the total balance.