watchtower/wtdb: return sorted ClientSession.CommittedUpdates

This commit replaces the map-based CommittedUpdates field with a slice.
When reading from disk, these will already be sorted by bbolt, so the
client restore the updates as presented without needing to sort them
first.

Since the key in the map variant was the sequence number, we refactor
the CommittedUpdate struct to have a sequence number and an embedded
CommittedUpdateBody (which is equivalent to the old CommittedUpdate).
The database is then expected to populate the sequence number from the
key on disk.

Since the sequence number is now directly integrated in the
CommittedUpdate struct, this allow allows us to remove the now redundant
seqNum argument from CommitUpdate.

watchtower/wtdb/client_session.go

@@ -86,10 +86,10 @@ type ClientSession struct {
 	// specifies a reward output.
 	RewardPkScript []byte
 
-	// CommittedUpdates is a map from allocated sequence numbers to unacked
-	// updates. These updates can be resent after a restart if the update
-	// failed to send or receive an acknowledgment.
-	CommittedUpdates map[uint16]*CommittedUpdate
+	// CommittedUpdates is a sorted list of unacked updates. These updates
+	// can be resent after a restart if the updates failed to send or
+	// receive an acknowledgment.
+	CommittedUpdates []CommittedUpdate
 
 	// AckedUpdates is a map from sequence number to backup id to record
 	// which revoked states were uploaded via this session.
@@ -107,8 +107,21 @@ type BackupID struct {
 }
 
 // CommittedUpdate holds a state update sent by a client along with its
-// SessionID.
+// allocated sequence number and the exact remote commitment the encrypted
+// justice transaction can rectify.
 type CommittedUpdate struct {
+	// SeqNum is the unique sequence number allocated by the session to this
+	// update.
+	SeqNum uint16
+
+	CommittedUpdateBody
+}
+
+// CommittedUpdateBody represents the primary components of a CommittedUpdate.
+// On disk, this is stored under the sequence number, which acts as its key.
+type CommittedUpdateBody struct {
+	// BackupID identifies the breached commitment that the encrypted blob
+	// can spend from.
 	BackupID BackupID
 
 	// Hint is the 16-byte prefix of the revoked commitment transaction ID.