BUG-1275: optimize InstanceIdentifierForXmlCodec

First batch of optimizations:
- use Splitter instead of String.split()
- use characters instead of Strings
- use ThreadLocalRandom
- sue SchemaContext.findModuleByNamespace()

Various optimizations revolving around precompiling Patterns, using
characters instead of strings,

Change-Id: Id2befcc49d4346917a39d434b005b07fe3f50b7c
Signed-off-by: Robert Varga <rovarga@cisco.com>