Refactoring of web-sockets in RESTCONF RFC-8040

Refactoring of web-sockets in RESTCONF RFC-8040

- Notificator is removed and replaced by ListenersBroker; mods.:
  A. One stream-name can be associated only with one YANG
     notification listener, because it is sufficient (XML/JSON
     format is distinguished directly in stream name).
  B. YANG notification streams are divided from data-change event
     streams; refactored method signatures.
  C. Added synchronization (R/W locking that also differs between
     access to data-change and YANG notification listeners).
- Exposing of public subsriber methods to BaseListenerInterface.
- Refactoring of CreateStream utilities and RPC processors.
- Refactoring of SubsribeToStream utilities and RPC processors.

Change-Id: I521f3b893a5cfa53d59a438e871924c54d6f4d7d
Signed-off-by: Jaroslav Tóth <jtoth@frinx.io>