add removal of CakeHelper in migration guide

related to cakephp/cakephp@6441e41
add detail on future plugin
fix typo
Add reasoning for cachehelper removal.
Update TOC.
Add a link for ESI.
Merge pull request #1514 from cakephp/3.0-cache-helper

3.0 cache helper
Put ConnectionManager in the right namespace.

Refs cakephp/cakephp#4071