Make the concept of namespaces explicit in dynamic attributes

This is in order to reduce the likelihood of naming clashes, make it easier to filter in/out attributes, ensure uniformity, etc.
All dynattrs in the opensim distro itself or likely future ones should be in the "OpenSim" namespace.
This does alter the underlying dynattrs data structure.  All data in previous structures may not be available, though old structures should not cause errors.
This is done without notice since this feature has been explicitly labelled as experimental, subject to change and has not been in a release.
However, existing materials data is being preserved by moving it to the "Materials" store in the "OpenSim" namespace.