Disconnect HTTPServer from RequestDispatcher

Disconnect HTTPServer from RequestDispatcher

RequestDispatcher is useless layering violation sitting between Netty
pipeline and RESTCONF protocol implementation.

Do not use RequestDispatcher in HTTPServer and instead use
onTransportChannelEstablished() to wire a RestconfSession into the
pipeline.

This means we have centralized protocol-level initialization at the
correct layer and a proper class interfacing between a particular Netty
channel and the RestconfServer.

RestconfRequestDispatcher now becomes a private implementation detail,
acting as a the common router between RestconfSession and
RestconfServer (et al.).

A follow-up patch will deal with exposing RestconfSession from
NettyServerRequest.

JIRA: NETCONF-1379
Change-Id: Ie24e440dbf24a850428902076175fb569981d0d9
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>