Fix org.jboss.weld.environment.deployment.WeldDeployment.loadBeanDeploymentArchive()

The current implementation does not fully comply with org.jboss.weld.bootstrap.spi.Deployment.loadBeanDeploymentArchive(Class<?>) contract:

If the deployment archive containing the given class is not currently a bean deployment archive, it must be added to the bean deployment archive graph and returned.

This does not happen if isolation is disabled or if there is only one bean deployment archive - right now, we return the only bean deployment archive we have and don't modify it in any way.

We should also synchronize the access to the shared additional bean deployment archive to prevent race conditions.