Split out operation output serialization

Split out operation output serialization

NormalizedNodePayload is quite overloaded, handling a ton of different
options.

This patch renames AbstractBody to RequestBody and creates its
counterpart ReplyBody. The JAX-RS serialization bit is handled through a
pair of MessageBodyWriters, which end up just calling ReplyBody methods.

We then define an OperationOutputBody, which handles the case of
RPC/action invocation result and stop using NormalizedNodePayload for
that purpose.

This also speeds up the entire process, as we can reuse JSON codecs
and not look them up again.

JIRA: NETCONF-773
Change-Id: Ib711451498f9bad485b69d14f6ae8195038ff506
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>