part of HADOOP-18103.
Add support for multiple ranged vectored read api in PositionedReadable.
The default iterates through the ranges to read each synchronously,
but the intent is that FSDataInputStream subclasses can make more
efficient readers especially in object stores implementation.
Also added implementation in S3A where smaller ranges are merged and
sliced byte buffers are returned to the readers. All the merged ranged are
fetched from S3 asynchronously.
Contributed By: Owen O'Malley and Mukund Thakur
