MODE-2271 Updated & fixed the existing performance tests
MODE-2271 Added a new "lazy" child references implementation which will only load & cache a list of children the first time it's being asked for a child. The child references are loaded & cached in a Medium instance, which has been simplified as far retrieving SNS indexes for ChildReferences because the Medium can calculate up-front the entire SNS indexes and store them on the ChildReference instance.
MODE-2271 Fixed the logic which processed child reorderings and update the parent document, since the previous code would cause the same child (children) to be added over and over again in the parent CHILDREN array, if the parent allowed SNS. The problem wasn't visible to client because of the "hack" for https://issues.jboss.org/browse/MODE-2120.
MODE-2271 Removed LazyMedium and moved the logic of loading/parsing the child references into Medium
Merge pull request #1199 from hchiorean/MODE-2271-3.x

MODE-2271-3.x Ported & adapted the code from MODE-2271 to the 3.x branch