Included a 'Caching' section to the Identity chapter

New section for 'Caching' with subsections for tokens and
token validation and assignment using CRUD.

Change-Id: Id68ec3b6e5dfa20123807ffa42a19c79c3123b41
Closes-bug: #1216478
Closes-bug: #1218194
Merge "Included a 'Caching' section to the Identity chapter"