Base class for enforcing the complete access of a file in one accessor.
Since we are not accessing the file using the splittable API, but instead are
using the "simple" stream API, it means that we cannot fetch different parts
(splits) of the file in different segments. Instead each file access brings
the complete file. And, if several segments would access the same file, then
each one will return the whole file and we will observe in the query result,
each record appearing number_of_segments times. To avoid this we will only
have one segment (segment 0) working for this case - enforced with
isWorkingSegment() method. Naturally this is the less recommended working
mode since we are not making use of segment parallelism. HDFS accessors for
a specific file type should inherit from this class only if the file they are
reading does not support splitting: a protocol-buffer file, regular file, ...