Optimize NetconfMessageTransformer.toRpcRequest()

Optimize NetconfMessageTransformer.toRpcRequest()

YangConstants provides a utility input QName factory method,
which is slighty more efficient than QName.create(), hence we use
that. Since the resulting QName is only used for lookups, we do
not intern it (it is short-lived anyway).

When we are determining the RPC definition, we unnecessarily
consult mappedRpcs twice -- store the result in a local variable
to make it a bit faster.

Change-Id: Iffa1f50af55b18a60c4adef17221f41661a278ec
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>