RFR: 8241837: Cleanup stringStream usage in ObjectSynchronizer

[Claes Redestad]

Hi,

this removes one unused stringStream, and refactors two other
so that they're only defined in the slow path error where they are used:

http://cr.openjdk.java.net/~redestad/8241837/open.00/

While a very small startup performance gain (removes ~50k instructions
on a Hello World), these stringStreams mallocs on init, which could be
problematic at times in this particular area.

Thanks!

/Claes