optimize metadata read

python need the "string" part of metadata only when cursor method .description() is called.

It seems to me that those string are always read by c/c : https://github.com/MariaDB/mariadb-connector-c/blob/master/libmariadb/mariadb_lib.c#L782
If this is the case, keeping the buffer then creating string only when asked might speed up result-set.
(7% perf improvement when querying mysql.user table on java connector).