wtclient: don't unnecessarily load exhausted sessions

In this commit, a new `ExhaustedSessionFilter` function is added and
used as a PostEvalFilterFn used when loading sessions from the DB. It
allows us to not unnecessarily load exhausted sessions into memory for
areas of the code where they will not be needed.

watchtower/wtclient/client.go

@@ -74,6 +74,14 @@ func (c *TowerClient) genSessionFilter(
 	}
 }
 
+// ExhaustedSessionFilter constructs a wtdb.ClientSessionFilterFn filter
+// function that will filter out any sessions that have been exhausted.
+func ExhaustedSessionFilter() wtdb.ClientSessionFilterFn {
+	return func(session *wtdb.ClientSession) bool {
+		return session.SeqNum < session.Policy.MaxUpdates
+	}
+}
+
 // RegisteredTower encompasses information about a registered watchtower with
 // the client.
 type RegisteredTower struct {
@@ -414,6 +422,7 @@ func New(config *Config) (*TowerClient, error) {
 		wtdb.WithPreEvalFilterFn(c.genSessionFilter(true)),
 		wtdb.WithPerMaxHeight(perMaxHeight),
 		wtdb.WithPerCommittedUpdate(perCommittedUpdate),
+		wtdb.WithPostEvalFilterFn(ExhaustedSessionFilter()),
 	)
 	if err != nil {
 		return nil, err
@@ -1627,6 +1636,7 @@ func (c *TowerClient) handleNewTower(msg *newTowerMsg) error {
 	sessions, err := getClientSessions(
 		c.cfg.DB, c.cfg.SecretKeyRing, &tower.ID,
 		wtdb.WithPreEvalFilterFn(c.genSessionFilter(true)),
+		wtdb.WithPostEvalFilterFn(ExhaustedSessionFilter()),
 	)
 	if err != nil {
 		return fmt.Errorf("unable to determine sessions for tower %x: "+