Expose streams with all supported encodings

Expose streams with all supported encodings

The entire idea of output-type for a stream is wrong. The user can
choose which encoding to pick based on operational datastore -- and
there we expose all encodings.

This requires a rather major refactor to deal with stream lifecycle:
- RestconfStream is the central point
- RestconfStream.Source is a source of events -- notifications, DTCL
- RestconfStream.Source.start() is invoked when the first subscriber
  arrives
- RestconfStream.Source is stopped when the last subscriber disappears
- both sides can terminate the stream

JIRA: NETCONF-1102
Change-Id: I6f7a196195ac96706c3c6b1797350e5fd96b261e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>