Introducing fs.azure.readahead.range parameter which can be set by the user.
Data will be populated in buffer for random reads as well which leads to fewer
remote calls.
This patch also changes the seek implementation to perform a lazy seek. The
actual seek is done when a read is initiated and data is not present in the buffer else
data is returned from the buffer thus reducing the number of remote storage calls.
Contributed By: Mukund Thakur
Change-Id: Ib920eedd0087caa150afa4d4c23e89df56b29e83
e8f9af6f2a