This addresses HADOOP-18521, "ABFS ReadBufferManager buffer sharing
across concurrent HTTP requests" by not trying to cancel
in progress reads.
It supercedes HADOOP-18528, which disables the prefetching.
If that patch is applied *after* this one, prefetching
will be disabled.
As well as changing the default value in the code,
core-default.xml is updated to set
fs.azure.enable.readahead = true
As a result, if Configuration.get("fs.azure.enable.readahead")
returns a non-null value, then it can be inferred that
it was set in or core-default.xml (the fix is present)
or in core-site.xml (someone asked for it).
Contributed by Pranav Saxena.
c67c2b7569