RESTCONF RFC8040 compliance: SSE support

Currently RESTCONF notifications are delivered only over Websocket. According to RFC8040[1] notifications must be sent over Server-Sent Events[2]

[1] https://tools.ietf.org/html/rfc8040#section-3.8
[2] https://www.w3.org/TR/2015/REC-eventsource-20150203/

how to test this:

1)./target/assembly/bin/client "feature:install odl-mdsal-model-inventory odl-netconf-mdsal odl-restconf odl-netconf-topology odl-netconf-connector-all odl-restconf-nb-rfc8040"

2)from postman collection call "1. create" and "2.get"

3)connect to url for example with curl :

curl -v -X GET http://127.0.0.1:8181/rests/notif/data-change-event-subscription/opendaylight-inventory:nodes/datastore=CONFIGURATION/scope=BASE -H "Content-Type: text/event-stream" -H "Authorization: Basic YWRtaW46YWRtaW4="

4) 3.update and 4. delete configuration and you must see data in SSE channel