gRpc connection error on filer when no volume left #2243
gRpc connection error on filer when no volume left #2243

fix https://github.com/chrislusf/seaweedfs/issues/2243

grpc do not cache connections only when connection problem happens.
Normal error results should not close the shared grpc connection.

chrislusf/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs,chrislusf/seaweedfs