ObjectIn/OutputStream improvements

[Chris Hegarty]

Thanks stuart, Mike, and Paul.

> - Why not have getClassSignature() return an interned string? (that's if interning is actually essential. Are we sure it's not just overhead?)

I didn’t want to change the existing use of interning here, just refactor the code a little to make it cleaner.

> I think that would be a better way to spend our performance investigation budget. Thus, it might be better to remove the StringBuilder field from this patch and submit another one focusing on UTF decoding; the other changes in the patch look good.

Agreed. This could be looked at separately.

Latest webrev:
  http://cr.openjdk.java.net/~chegar/serial_stupp.01/

Thanks,
-Chris.