Prepared statements can return multiple result sets since MySQL 5.5. That can happen when calling stored procedures using `CALL x();`.
This change removes the previous caching of result bindings, since the number of fields in a result can differ from result set to result set. This could potentially have a negative impact on prepared statements always only returning one result set, since the result binding buffers are recreated everytime the statement is executed instead of once. That difference should be negligible.
Fixes#823.
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