ZOOKEEPER-3585:[WIP]Add a documentation about RequestProcessors

[maoling]

• this is still a work in the progress,Plz don't review before removing the WIP tag.
• more details in the ZOOKEEPER-3585

[anmolnar]

I always support adding documentation, but fyi RequestProcessors are very well documented in the book.

[maoling]

@anmolnar

• but fyi RequestProcessors are very well documented in the book.

  Which book? Could you plz share us with the book name?

• The structure of request processor chain has not changed, but some important improvements had done these years especially for CommitProcessor(e.g: ZOOKEEPER-2024), we will show them in a detailed and understanding way in that documentation work.

[eolivelli]

@maoling AFAIK there is only ONE great book about Zookeeper :) Search Zookeeper on Amazon

[maoling]

@eolivelli My aim is making the official documentation better than any book:)

[anmolnar]

@maoling Sure. Please let me know if you're done.