OAK-619 - Lock-free MongoMK implementation

 Adding initial support for node deletion. Deleted nodes have '_deleted' set to true in versioned manner and retrieval logic check for this property for finding valid nodes.

 This impl though would not work well for deletion of deep node hierarchies.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1449018 13f79535-47bb-0310-9956-ffa450edef68