optimize receiving metadata from the server

in 10.6 the server only sends metadata to the client if it has changed since the last stmt_execute call.

C/C supports that, of course. But in a rather inefficient way, with too much copying of metadata, which, quite possibly, destroys much of the effect of the optimization.

This task is about verifying the claim above (does C/C implementation indeed adversely affects the performance), and optimizing it, if needed.