Revisit DataContainerCodecPrototype.instance locking

As noted in the documentation around the field, it is an utterly hot reference. Once we have dealt with MDSAL-578, we should be at a place where we are dealing with safe publishing of an effectively-immutable object in the instance field.

Re-examine referenced objects and determine if that is really the case. If it is, consider:

• concurrent loading, i.e. dropping 'synchronized' from loadInstance()
• using a weaker initial access, based on http://gee.cs.oswego.edu/dl/html/j9mm.html, i.e. opaque or even plain if possible