Improvements to Stampede Test

RandomWalkTest should:
1) include an action that causes external nodes and their children to be loaded on a grand scale. This will cause the SharedItemStateManager to exhaust its ItemState cache (items will be evicted)
2) register an event listener for every session that is active. This will more closely simulate typical CMS use cases where observers are registered all the time.
3) include a delete document action that deletes a random document from the repository
4) inlcude an action that adds a link to a random document to a random document