Do not use anymore shaded version of wagon-http as it we can control commons-logging version

Integrated the fix toward 1.521.

Integrated the new version of remoting

The plan of the record is that we start producing 1.6 class files and
see if people start complaining. If we hear from users, we'll re-visit
our decision, and if we don't, then we'll start using 1.6 features
everywhere in the code.

- integrated the newer release of remoting
- jar caching won't work with class file directory, so plugin
  WEB-INF/classes are now exploded as WEB-INF/lib/classes.jar
  (This should also solve the problem of slow plugin extraction in the
  presene of Anti-virus software on Windows.)
- because the structure of the exploded jar file has changed, I changed
  the up-to-date check timestamp file name to force re-extraction in
  existing installations.

This rule enforces a bytecode version in dependencies.
This would prevent cases like JENKINS-8914 where someone had
introduced a dependency compiled with JDK1.6 at a time where
Jenkins was still on 1.5.

With 1.7 already here and 1.8 coming, this should help no to do that
again.