BUG-1281: optimize JSON codec

BUG-1281: optimize JSON codec

Multiple optimizations made in one go:

- do not use string concatenation for LOG.debug()
- do not use string concatenation in Preconditions.checkState()
- methods which do not need state are made static
- use chars instead of strings where possible
- do not instantiate ArrayList needlessly to access first element in a set
- optimize namespace/localname parsing by not using String.split()
- use Charset constant instead of charset name

Change-Id: If1c4ea50d7d1d2aae4f52d1dc68ff79b901208b0
Signed-off-by: Robert Varga <rovarga@cisco.com>