code: private ByteBuffer convertValueByFunction(Tree functionCall, CfDef columnFamily, ByteBuffer columnName)
      {
          return convertValueByFunction(functionCall, columnFamily, columnName, false);
      }
docs: Used to convert value (function argument, string) into byte[]
  calls convertValueByFunction method with "withUpdate" set to false
  @param functionCall - tree representing function call ^(FUNCTION_CALL function_name value)
  @param columnFamily - column family definition (CfDef)
  @param columnName   - also updates column family metadata for given column
  @return byte[] - string value as byte[]