fix STMBench7 perf hack under Scala 2.10

Summary: Scala 2.9's TreeMap generated a lot of garbage during
iteration, so I hacked an efficient custom iteration using reflection
and direct access to encapsulated types.  Scala 2.10 fixes the perf
problem while changing the implementation, so the hack goes from helping
to breaking.  This diff falls back to the stock iterator for 2.10 or
higher.