WL#5991: Cluster: Document NDB API Counters & Statistics

Summary
 - New activity counters added to NdbApi, and exposed in MySQLD via
SHOW STATUS command
 - Useful for :
     - query debugging and optimisation
     - understanding performance
     - server monitoring
 - Pushed to 7.0.22, 7.1.11.

Patches :
http://lists.mysql.com/commits/130471
http://lists.mysql.com/commits/130841

See attached text file for details.

I'll do this in two parts:

Part I: Add basic documentation for all the new status variables; add section to
Cluster Management with overview of counters and corresponding status vars; some
basic SHOW/SELECT statements.

Part II: A tutorial with more detailed examples. (Possibly repurpose this as a
DevZone article if Frazer is agreeable?)