Fixes in \copy implementation for Clickhouse

[murfffi]

A couple of fixes in the Copy function for Clickhouse:

• Don't prepare the SELECT statement that determines columns for the destination table as the Clickhouse driver omly supports preparing INSERT statements
• De-reference the pointers populated by Scan before supplying them to arguments of Exec because Clickhouse, in some cases, doesn't accept a pointer to an arguments, instead of the argument itself. For example, this happens when source database driver doesn't support ScanType() and the value populated by Scan is interface{}.

The new automated test fails without the fixes and succeeds with them.

[murfffi]

The automated test could have been either in driver_test.go or in clickhouse_test.go . The copy test in driver_test.go expects destination DB supports RowsAffected but Clickhouse doesn't. So I made a dedicated copy test in clickhouse_test.go. I can implement the other way but the code in drivers_test.go can become convoluted.