WL#1256: NDB Cluster Table Handler Documentation and Improvements

Affects: Server-7.1   —   Status: Assigned   —   Priority: Medium

Write up a documentation of how the NDB Cluster Table Handler works.
Also write up improvements on how it can be improved for better performance.

Documentation will hopefully be useful for any future new table handlers
to be added.

Main reason to document and provide good material for describing what
improvements we need to add to the table handler interface to improve the
discussion on these improvements.