Bug 499: Initial implementation of supporting tree structures

Bug 499: Initial implementation of supporting tree structures

  - StoreTreeNode - interface defining common interface to tree
    structures used by data store.

  - TreeNodeUtils - utility methods which uses StoreTreeNode
      as tree definitions and provides functionality such
      as retrieval of deep nodes.

  - StoreMetadataNode - immutable tree node (and tree), representing
        versioning metadata and data.

  - NodeModification - mutable tree node (and tree),
       representing modification (diff) which should be applied to
       data tree. Tree is lazily populated by invoking client
       operations to be maded.

This patchset requires https://git.opendaylight.org/gerrit/#/c/5646/
to be merged.

Change-Id: I2d2437ea64ba54565ba60826fe0e7c14f071598e
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>