Review request for JDK-8046201: Avoid repeated flattening of nested ConsStrings

[Hannes Wallnoefer]

Please review JDK-8046201: Avoid repeated flattening of nested ConsStrings:
http://cr.openjdk.java.net/~hannesw/8046201/

Note that I made ConsString.flatten() synchronized. I didn't observe any 
performance degradation.

Thanks,
Hannes