Populate restconf-state/streams

Populate restconf-state/streams

ListenersBroker needs to manage restconf-state/streams, so users
can locate the endpoint servicing each stream.

Pick up the required pieces from RestconfStateStreams and surrounding
commented-out code to generate proper entries.

Also clean up tests, so that we have nice and crispy test suite, without
duplications and asserting proper changes.

JIRA: NETCONF-1102
Change-Id: Ifddb9fba119e6580ac9cb1c6377b63285d8d6cb0
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>