Avoid creating unnecessary type metadata instances; only init once per column.

Inside RecordsStreamProducer#schemaChanged we make several calls to the ReplicationMessage.Column#getTypeMetadata() which will construct a new TypeMetadataImpl for each call. We likely should only create the object once when it does not yet exist, e.g.:

@Override
public TypeMetadataImpl getTypeMetadata() {
  if (typeMetadata == null) {
    initMetadata();
  }
  return typeMetadata;
}