[Hadoop Wiki] Update of "ZooKeeper/ZabProtocol" by BenjaminReed

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The following page has been changed by BenjaminReed:
http://wiki.apache.org/hadoop/ZooKeeper/ZabProtocol

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  We would like to modularize the Atomic Broadcast layer of ZooKeeper so that it can be used independently. It also help to be able to test more effectively.
  We have named the atomic broadcast layer Zab, after the river in the middle east [http://en.wikipedia.org/wiki/Zab], to distinguish it from ZooKeeper as a whole.
- To help extract Zab from ZooKeeper we have opened the ZOOKEEPER-30 Jira and we have attached attachment:zookeeper-internals.pdf which, although old, shows how we connect RequestProcessors together.
+ To help extract Zab from ZooKeeper we have opened the ZOOKEEPER-30 Jira and we have attached attachment:zookeeper-internals.pdf and attachement:zookeeper-impl.pdf which, although old, shows how we connect RequestProcessors together.
  The actual RequestProcessor chain is setup in the {{{setupRequestProcessors}}} method of ZooKeeperServer, FollowerZooKeeperServer, and LeaderZooKeeperServer.
  
  Zab is a standard atomic broadcast layer, but there are somethings that ZooKeeper uses from Zab that is specific to its implementation: