Performance improvement: Lazy evaluation of Velocity context

One of the most significant items in the profiled performance builds is in the ConfluenceVelocityManager.addContainerContexts(...) method - specifically in the calls into Spring.

In some of the profiles it takes 6% of runnable time and 13% of blocked time.

As a lot of these container contexts are not required, lazily requesting them when actually required should significantly reduce pressure on Spring.