RFR: 8188858: Caching latestUserDefinedLoader() results in ObjectInputStream.readObject()

Hi all, Please review a change for JDK-8188858. Bug report: https://bugs.openjdk.java.net/browse/JDK-8188858 Webrev: http://cr.openjdk.java.net/~horii/8188858/webrev.00/ This change caches the result of latestUserDefinedLoader() when objects are deserialized, so the decerializer can avoid redundant stack walking to resolve classes of deserializing objects. This change improved maxjOPS of SPECjbb2015 by 6% on a POWER8 machine. Regards, Ogata