Cassandra: Use TokenAwareHostPolicy by default with fallback

See https://pkg.go.dev/github.com/gocql/gocql#hdr-Data_center_awareness_and_query_routing

weed/command/scaffold.go

@@ -217,6 +217,8 @@ username=""
 password=""
 # This changes the data layout. Only add new directories. Removing/Updating will cause data loss.
 superLargeDirectories = []
+# Name of the datacenter local to this filer, used as host selection fallback.
+localDC=""
 
 [hbase]
 enabled = false

weed/filer/cassandra/cassandra_store.go

@@ -32,6 +32,7 @@ func (store *CassandraStore) Initialize(configuration util.Configuration, prefix
 		configuration.GetString(prefix+"username"),
 		configuration.GetString(prefix+"password"),
 		configuration.GetStringSlice(prefix+"superLargeDirectories"),
+		configuration.GetString(prefix+"localDC"),
 	)
 }
 
@@ -40,13 +41,19 @@ func (store *CassandraStore) isSuperLargeDirectory(dir string) (dirHash string,
 	return
 }
 
-func (store *CassandraStore) initialize(keyspace string, hosts []string, username string, password string, superLargeDirectories []string) (err error) {
+func (store *CassandraStore) initialize(keyspace string, hosts []string, username string, password string, superLargeDirectories []string, localDC string) (err error) {
 	store.cluster = gocql.NewCluster(hosts...)
 	if username != "" && password != "" {
 		store.cluster.Authenticator = gocql.PasswordAuthenticator{Username: username, Password: password}
 	}
 	store.cluster.Keyspace = keyspace
+	fallback := gocql.RoundRobinHostPolicy()
+	if localDC != "" {
+		fallback = gocql.DCAwareRoundRobinPolicy(localDC)
+	}
+	store.cluster.PoolConfig.HostSelectionPolicy = gocql.TokenAwareHostPolicy(fallback)
 	store.cluster.Consistency = gocql.LocalQuorum
+
 	store.session, err = store.cluster.CreateSession()
 	if err != nil {
 		glog.V(0).Infof("Failed to open cassandra store, hosts %v, keyspace %s", hosts, keyspace)