watchtower/multi: embed TxPolicy in wtpolicy.Policy

This commit splits out the parameters that shape the justice transaction
into their own struct, which then embedded within the overarching
wtpolicy.Policy which may have additional parameters describing
operation of the session.

This is done as a preliminary step to support comparison of sessions
based on matching TxPolicy configurations. This prevents otherwise
identical Policies from being counted as different if operational
parameters like MaxUpdates differ, even if it has no material difference
to the justice transaction.

watchtower/wtserver/create_session.go

@@ -89,11 +89,13 @@ func (s *Server) handleCreateSession(peer Peer, id *wtdb.SessionID,
 	info := wtdb.SessionInfo{
 		ID: *id,
 		Policy: wtpolicy.Policy{
-			BlobType:     req.BlobType,
-			MaxUpdates:   req.MaxUpdates,
-			RewardBase:   req.RewardBase,
-			RewardRate:   req.RewardRate,
-			SweepFeeRate: req.SweepFeeRate,
+			TxPolicy: wtpolicy.TxPolicy{
+				BlobType:     req.BlobType,
+				RewardBase:   req.RewardBase,
+				RewardRate:   req.RewardRate,
+				SweepFeeRate: req.SweepFeeRate,
+			},
+			MaxUpdates: req.MaxUpdates,
 		},
 		RewardAddress: rewardScript,
 	}