S3A input stream support for the few fs.option.openfile settings.
As well as supporting the read policy option and values,
if the file length is declared in fs.option.openfile.length
then no HEAD request will be issued when opening a file.
This can cut a few tens of milliseconds off the operation.
The patch adds a new openfile parameter/FS configuration option
fs.s3a.input.async.drain.threshold (default: 16000).
It declares the number of bytes remaining in the http input stream
above which any operation to read and discard the rest of the stream,
"draining", is executed asynchronously.
This asynchronous draining offers some performance benefit on seek-heavy
file IO.
Contributed by Steve Loughran.
Change-Id: I9b0626bbe635e9fd97ac0f463f5e7167e0111e39
e0cd0a82e0