Enhance RESTconf to implement latest draft-ietf-restconf-netconf

Can we please enhance the RESTconf server in ODL Beryllium to implement draft-ietf-restconf-netconf-04 rather than draft-bierman-restconf-netconf-02?

draft-ietf-restconf-netconf-04 is in WG last call so hopefully will progress soon to RFC status. Of course we'll need to track any changes between now and then.

2 key areas to address in the enhancement:

1) the data store access URLs change from "config" and "operational" to "data". We may wish to keep config/operational for a transition period. For e.g. NETCONF we can simply map the RESTconf GET on a "data" URL to a NETCONF "get" as we currently do for operational URLs. Where the URL references config data in the mounted datastore that will give the same result as we get today with GET on both operational and config URLs for that datastore (as a "get" on config data is equivalent to a "get-config" on the running datastore). But for access to ODL's internal data-store things may change. Currently, for example, operational and config GETs on the node inventory give different results.

2) support for notifications as per section 6 of the draft. There potentially a few use cases here:
i) ability to subscribe directly to notifications in mounted data-stores.
ii) ability to subscribe to notifications carried over the ODL message bus (which is introduced in Lithium).
iii) ability to subscribe to notifications sent directly from applications to the client.