Document extended buffer pool design

The principles of buffer pool extension design proposal are currently dispersed between Jira, code comments, conversations and undocumented parts which can only be inferred from reading the code. In order to reasonably progress, a singlee document should be created to cover:

• The overall design
• The shortcuts taken by the existing prototype
• Proposed final design (what should replace the shortcuts)

As part of this a presentation may be created to facilitate discussion of the design.