Convert anyxml nodes lazily

Convert anyxml nodes lazily

The problem is talking about the direct creating of ContainerNode
from the Anyxml in the NetconfMessageTransformer. Later, while
the YT is parsing data, it expects the anyxml on that place
according to the schema context. In this way, we need to use
the AnyXml builder while the NetconfMessage is processing.

Transformation to a container-based layout is performed afterwards
as needed.

JIRA: NETCONF-616
Change-Id: I8ca6d93e42a845de67721209e5e6362c47703db9
Signed-off-by: Jakub Toth <jtoth@luminanetworks.com>
Signed-off-by: Miroslav Macko <mmacko@luminanetworks.com>