Utility to ensure that the migrations from balance() are consistent:
Tablet objects are not null
Source and destination tablet servers are not null and current
Allow the concrete servlet implementation to provide attributes on the body HTML tag,
such as 'onload', which can be used to call Javascript methods on page load.
this class just provides an interface to read from a MapFile mostly takes care of reporting start and end keys
need this because a single row extent can have multiple columns this manages all the columns (each handled by a store) for a single row-extent