watchtower: add PreEvaluateFilterFn callback

In this commit, a PreEvaluateFilterFn option is added to the
wtdb.ClientSessionListCfg and it is used instead of a separate
ClientSessionFilterFn parameter. This neatens quiet a few function
signatures.

watchtower/wtclient/interface.go

@@ -60,8 +60,7 @@ type DB interface {
 	// ListClientSessions returns the set of all client sessions known to
 	// the db. An optional tower ID can be used to filter out any client
 	// sessions in the response that do not correspond to this tower.
-	ListClientSessions(*wtdb.TowerID, wtdb.ClientSessionFilterFn,
-		...wtdb.ClientSessionListOption) (
+	ListClientSessions(*wtdb.TowerID, ...wtdb.ClientSessionListOption) (
 		map[wtdb.SessionID]*wtdb.ClientSession, error)
 
 	// GetClientSession loads the ClientSession with the given ID from the