Fix ISC user context entry multiple and incompatible uses

There is a conflict between extended info code which overrides the ISC entry of lease user context and limits hook which uses it in a different way.

Three possible solutions:

• use different entries e.g. ISC-extended-info and ISC-client-classes: efficient but not backward compatible.
• use sub-entries under ISC: not efficient and not backward compatible.
• fix extended info code with again three choices: just fix, move to different entries, move to sub-entries. Terribly inefficient during the transition but backward compatible.