Fixes #2953.

statements in a rewindable iterator. Skipped the test for iterating
multiple times with a message regarding that iterators are non
rewindable.

Only added to mysqli for now, can add to other drivers (assuming they
support reseting the pointer in some way) after feedback.

I don't really understand the mysqli driver too much and the changes
break the tests so undo using an iterator for this particular driver.

Allows result sets to be iterated over without running out of memory on
large data sets.