Do not instantiate NormalizedNodes for filter

Do not instantiate NormalizedNodes for filter

Using ImmutableNodes.fromInstanceId() does not work for leaf nodes,
as we do not have a value, which violates LeafNode contract -- leading
to an IllegalStateException when a user attempts to read() a single
leaf via a transaction.

As it turns out, this is completely unnecessary, as all we are doing
is iterating through the temporary NormalizedNode and creating
NormalizedNodeStreamWriter events based on that.

This patch replicates ImmutableNodes.fromInstanceId() logic, but
simplifies it for this particular task, short-cutting NormalizedNode
creation and emitting events directly as we are iterating over
the YangInstanceIdentifier.

JIRA: NETCONF-563
Change-Id: I0781e81d812ee979deaafb9dfa811e143866f4af
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>