Address various class-loader deadlock / sync issues

Address various class-loader deadlock / sync issues
https://issues.apache.org/bugzilla/show_bug.cgi?id=44041
https://issues.apache.org/bugzilla/show_bug.cgi?id=48694
https://issues.apache.org/bugzilla/show_bug.cgi?id=48903

Whilst parallel class-loading would be a nice feature, the various issues that have emerged have demonstrated that anything other than synchronized(this) is likely to cause issues.

Parallel class-loading will be explored for Tomcat 7 (disabled by default) and ported back to 6.0.x when proven to be stable.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@927565 13f79535-47bb-0310-9956-ffa450edef68