Upgrade to latest released Plexus

We're using an old version. By upgrading we hope to:

• get the perlookup bug fix (the instances are never released thus leading to memory leaks if the user doesn't explicitely release them)
• fix the pb of plexus threading we sometimes have at xwiki startup
• make our code ready for the new plexus (which uses xbean and guice)
• -use the new PlexusContainer.addComponent() which allows to register an implementation against a role. Strangely though it doesn't seem to be possible to register against a role+roleHint though- False alert it's using a component descriptor under the hood and is ignoring the implementation passed...

We're currently using a very old plexus (1.0-alpha-31 from 2007).