watchtower/wtdb: add bolt db impl of Queue

This commit adds a new generic DiskQueueDB type which is an
bbolt implementation of the Queue interface.

watchtower/wtdb/client_db.go

@@ -94,6 +94,10 @@ var (
 	// 	db-session-id -> last-channel-close-height
 	cClosableSessionsBkt = []byte("client-closable-sessions-bucket")
 
+	// cTaskQueue is a top-level bucket where the disk queue may store its
+	// content.
+	cTaskQueue = []byte("client-task-queue")
+
 	// ErrTowerNotFound signals that the target tower was not found in the
 	// database.
 	ErrTowerNotFound = errors.New("tower not found")
@@ -2060,6 +2064,15 @@ func (c *ClientDB) AckUpdate(id *SessionID, seqNum uint16,
 	}, func() {})
 }
 
+// GetDBQueue returns a BackupID Queue instance under the given namespace.
+func (c *ClientDB) GetDBQueue(namespace []byte) Queue[*BackupID] {
+	return NewQueueDB[*BackupID](
+		c.db, namespace, func() *BackupID {
+			return &BackupID{}
+		},
+	)
+}
+
 // putChannelToSessionMapping adds the given session ID to a channel's
 // cChanSessions bucket.
 func putChannelToSessionMapping(chanDetails kvdb.RwBucket,