Rework how JS's oo1.DB.exec() flags its Stmt objects to make certain Stmt APIs illega...

Rework how JS's oo1.DB.exec() flags its Stmt objects to make certain Stmt APIs illegal (i.e. throwing) if called while that Stmt is being step()ped by DB.exec() (which can happen via client-provided per-result-row callbacks). This is an internal change only - the API is unaffected. Remove some unrelated dead code.

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