watchtower: add ListClosableSessions method

This commit adds a new ListClosableSessions method to the tower client
DB. This method will return a map of sessionIDs to block heights. The
IDs belong to sessions that are considered closable and the block
heights are the block height at which the last associated channel for
the session was closed in.

watchtower/wtmock/client_db.go

@@ -551,6 +551,20 @@ func (m *ClientDB) AckUpdate(id *wtdb.SessionID, seqNum,
 	return wtdb.ErrCommittedUpdateNotFound
 }
 
+// ListClosableSessions fetches and returns the IDs for all sessions marked as
+// closable.
+func (m *ClientDB) ListClosableSessions() (map[wtdb.SessionID]uint32, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	cs := make(map[wtdb.SessionID]uint32, len(m.closableSessions))
+	for id, height := range m.closableSessions {
+		cs[id] = height
+	}
+
+	return cs, nil
+}
+
 // FetchChanSummaries loads a mapping from all registered channels to their
 // channel summaries.
 func (m *ClientDB) FetchChanSummaries() (wtdb.ChannelSummaries, error) {