Persistent PDO resources might be wrong w.r.t. PDO::ATTR_STATEMENT_CLASS

According to the PHP manual, persistent PDO resources don't support PDO_ATTR_STATEMENT_CLASS (http://php.net/manual/en/pdo.setattribute.php).

HHVM's implementation of PDO seems to want to agree with this behavior, since there's a check for an `is_persistent' in the implementation of setattribute; however, that flag is not actually ever set anywhere.

This shouldn't be too complicated to resolve:
1/ Make sure PHP5's behavior agrees with the doc; if not, we can probably just keep the behavior also.
2/ If so, check for persistence another way (or set `is_persistent').
3/ A bunch of cleanup is possible if we match this behavior; e.g., we can probably kill persistent saving/restoring of the actual PDOResource objects, we can move the clsname field out of PDOConnection and into PDOResource as a runtime data structure, etc.