S3AFileSystem.listStatus() is optimized for invocations
where the path supplied is a non-empty directory.
The number of S3 requests is significantly reduced, saving
time, money, and reducing the risk of S3 throttling.
Contributed by Mukund Thakur.
Change-Id: I7cc5f87aa16a4819e245e0fbd2aad226bd500f3f
7e642ec5a3