Javadoc
Handle subclass specific set up.
Each of the entry points used by the MapReduce framework,
#createRecordReader(InputSplit,TaskAttemptContext) and
#getSplits(JobContext),
will call
#initialize(JobContext) as a convenient centralized location to handle
retrieving the necessary configuration information and calling
#initializeTable(Connection,TableName).
Subclasses should implement their initialize call such that it is safe to call multiple times.
The current TableInputFormatBase implementation relies on a non-null table reference to decide
if an initialize call is needed, but this behavior may change in the future. In particular,
it is critical that initializeTable not be called multiple times since this will leak
Connection instances.