Fix a couple of test error messages
Don't bypass the prepared statement in stmt.Exec()

I fixed the Query case in 59dc557f48025f2793e034756cea8062643c53ca, but
it apparently didn't go the entire way.  Using the already-prepared
statement is both less surprising, and also *faster* than running
simpleExec().

This also means that there's no need to store the query inside the
"stmt" struct anymore, so get rid of it.
Refactor writeBuf
Decouple rows and stmt

We need the ability to return rows objects which don't have an
underlying prepared statement.
1rt wip
Apparently this turned into a bigger surgery
This is probably what this should look like
Wasn't the previous coding a bit silly?
Get rid of cn.binaryModeQuery