Do not format QNames to string on input

Do not format QNames to string on input

Profiling has revelead the creation of String for QNameFactory
lookup is taking around 19% of overall cost of
NormalizedNodeInputStreamReader.readQName().

Rather than using concatenation, use a dedicated Key object, which
holds the separated-out localname/namespace/revision strings,
thus allocating a small object instead of a full-blown String.

This improves snapshot deserialization perfomance on a 350MiB
snapshot by about 16%, also resulting in lower memory pressure.

JIRA: CONTROLLER-1897
Change-Id: If3e9e4b332b0a5acbe33aa10fca189bbc7da8c81
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>