watchtower: remove CommittedUpdates from ClientSession

In this commit, the new ListClientSession functional options and new
FetchSessionCommittedUpdates function are utilised in order to allow us
to completely remove the CommittedUpdates member from the ClientSession
struct.

watchtower/wtclient/session_queue.go

@@ -109,7 +109,9 @@ type sessionQueue struct {
 }
 
 // newSessionQueue intiializes a fresh sessionQueue.
-func newSessionQueue(cfg *sessionQueueConfig) *sessionQueue {
+func newSessionQueue(cfg *sessionQueueConfig,
+	updates []wtdb.CommittedUpdate) *sessionQueue {
+
 	localInit := wtwire.NewInitMessage(
 		lnwire.NewRawFeatureVector(wtwire.AltruistSessionsRequired),
 		cfg.ChainHash,
@@ -137,7 +139,7 @@ func newSessionQueue(cfg *sessionQueueConfig) *sessionQueue {
 
 	// The database should return them in sorted order, and session queue's
 	// sequence number will be equal to that of the last committed update.
-	for _, update := range sq.cfg.ClientSession.CommittedUpdates {
+	for _, update := range updates {
 		sq.commitQueue.PushBack(update)
 	}