watchtower/wtdb: create embedded ClientSessionBody

This commit splits out the portions of the ClientSession into an
embedded ClientSessionBody, since these fields will be serialized
together on-disk.

watchtower/wtclient/backup_task.go

@@ -126,8 +126,7 @@ func (t *backupTask) inputs() map[wire.OutPoint]input.Input {
 // SessionInfo's policy. If no error is returned, the task has been bound to the
 // session and can be queued to upload to the tower. Otherwise, the bind failed
 // and should be rescheduled with a different session.
-func (t *backupTask) bindSession(session *wtdb.ClientSession) error {
-
+func (t *backupTask) bindSession(session *wtdb.ClientSessionBody) error {
 	// First we'll begin by deriving a weight estimate for the justice
 	// transaction. The final weight can be different depending on whether
 	// the watchtower is taking a reward.

watchtower/wtclient/backup_task_internal_test.go

@@ -69,7 +69,7 @@ type backupTaskTest struct {
 	expSweepAmt      int64
 	expRewardAmt     int64
 	expRewardScript  []byte
-	session          *wtdb.ClientSession
+	session          *wtdb.ClientSessionBody
 	bindErr          error
 	expSweepScript   []byte
 	signer           input.Signer
@@ -205,7 +205,7 @@ func genTaskTest(
 		expSweepAmt:      expSweepAmt,
 		expRewardAmt:     expRewardAmt,
 		expRewardScript:  rewardScript,
-		session: &wtdb.ClientSession{
+		session: &wtdb.ClientSessionBody{
 			Policy: wtpolicy.Policy{
 				BlobType:     blobType,
 				SweepFeeRate: sweepFeeRate,

watchtower/wtclient/session_negotiator.go

@@ -417,14 +417,15 @@ func (n *sessionNegotiator) tryAddress(privKey *btcec.PrivateKey,
 			privKey.PubKey(),
 		)
 		clientSession := &wtdb.ClientSession{
-			TowerID:        tower.ID,
+			ClientSessionBody: wtdb.ClientSessionBody{
+				TowerID:        tower.ID,
+				KeyIndex:       keyIndex,
+				Policy:         n.cfg.Policy,
+				RewardPkScript: rewardPkScript,
+			},
 			Tower:          tower,
-			KeyIndex:       keyIndex,
 			SessionPrivKey: privKey,
 			ID:             sessionID,
-			Policy:         n.cfg.Policy,
-			SeqNum:         0,
-			RewardPkScript: rewardPkScript,
 		}
 
 		err = n.cfg.DB.CreateClientSession(clientSession)

watchtower/wtclient/session_queue.go

@@ -215,7 +215,7 @@ func (q *sessionQueue) AcceptTask(task *backupTask) (reserveStatus, bool) {
 	//
 	// TODO(conner): queue backups and retry with different session params.
 	case reserveAvailable:
-		err := task.bindSession(q.cfg.ClientSession)
+		err := task.bindSession(&q.cfg.ClientSession.ClientSessionBody)
 		if err != nil {
 			q.queueCond.L.Unlock()
 			log.Debugf("SessionQueue %s rejected backup chanid=%s "+