Updated RESTCONF implementation

Updated RESTCONF implementation

  - Restconf implementation now implements new SAL readign contracts
    in form of <String,CompositeNode> where String is Restconf version
    of instance identifier.

    Translation of Restconf InstanceIdentifier into Binding-Independent
    Restconf Instance-Identifier is done by ControllerContext class
    which implements stateful translation of identifier based on schema
    context.

    After translation of instence-identifier most of actions maps
    directly to Binding-Independent APIs.

Change-Id: I248470e282826a3f04f6e99f54aae8fd14ea2eb3
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>