[MNG-4632] Class loading is not thread-safe
[MNG-4632] Class loading is not thread-safe

Fixed problem where ClassWorld was missing a synchronized.

Also code-reviewed synchronization in ClassWorld vs ClassLoader and
discovered no other problems.

git-svn-id: d3061b52f177ba403f02f0a296a85518da96c313@933714 13f79535-47bb-0310-9956-ffa450edef68

vedmishr/demo1,karthikjaps/maven,lbndev/maven,olamy/maven,keith-turner/maven,dsyer/maven,Distrotech/maven,lbndev/maven,Tibor17/maven,njuneau/maven,cstamas/maven,cstamas/maven,pkozelka/maven,barthel/maven,stephenc/maven,likaiwalkman/maven,apache/maven,wangyuesong0/maven,stephenc/maven,changbai1980/maven,skitt/maven,kidaa/maven-1,mizdebsk/maven,xasx/maven,keith-turner/maven,mcculls/maven,wangyuesong/maven,dsyer/maven,wangyuesong0/maven,skitt/maven,wangyuesong0/maven,xasx/maven,cstamas/maven,likaiwalkman/maven,rogerchina/maven,stephenc/maven,Distrotech/maven,josephw/maven,trajano/maven,barthel/maven,xasx/maven,mcculls/maven,mizdebsk/maven,runepeter/maven-deploy-plugin-2.8.1,ChristianSchulte/maven,wangyuesong/maven,aheritier/maven,njuneau/maven,rogerchina/maven,Tibor17/maven,changbai1980/maven,Mounika-Chirukuri/maven,atanasenko/maven,changbai1980/maven,atanasenko/maven,Mounika-Chirukuri/maven,mizdebsk/maven,trajano/maven,Mounika-Chirukuri/maven,runepeter/maven-deploy-plugin-2.8.1,rogerchina/maven,apache/maven,keith-turner/maven,lbndev/maven,kidaa/maven-1,josephw/maven,ChristianSchulte/maven,apache/maven,karthikjaps/maven,atanasenko/maven,pkozelka/maven,kidaa/maven-1,olamy/maven,karthikjaps/maven,barthel/maven,aheritier/maven,gorcz/maven,josephw/maven,pkozelka/maven,skitt/maven,gorcz/maven,likaiwalkman/maven,mcculls/maven,wangyuesong/maven,gorcz/maven,njuneau/maven,dsyer/maven,ChristianSchulte/maven,vedmishr/demo1,vedmishr/demo1,trajano/maven,olamy/maven,aheritier/maven