Send Xclose commands to avoid server side space leaks

[joerivanruth]

The server automatically discards the result set if it was small enough to send everything immediately, as configured with Xreply_size. Larger result sets are kept on the server so the client can request the remaining rows later using Xexport. It this situation, the client must explicitly destroy the result set using Xclose.

[joerivanruth]

There seems to be a number of test failures but they appear to have nothing to do with this commit..