part of HADOOP-18103.
Also introducing a config fs.s3a.vectored.active.ranged.reads
to configure the maximum number of number of range reads a
single input stream can have active (downloading, or queued)
to the central FileSystem instance's pool of queued operations.
This stops a single stream overloading the shared thread pool.
Contributed by: Mukund Thakur
Conflicts:
hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/Constants.java
hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/S3AFileSystem.java
bbe841e601