Improve getSplits() performance for fs implementations that can utilize performance gains from recursive listing

FileInputFormat (both mapreduce and mapred implementations) use recursive listing while calculating splits. They however do this by doing listing level by level. That means to discover files in /foo/bar means they do listing at /foo/bar first to get the immediate children, then make the same call on all immediate children for /foo/bar to discover their immediate children and so on. This doesn't scale well for object store based fs implementations like s3 and swift because every listStatus call ends up being a webservice call to backend. In cases where large number of files are considered for input, this makes getSplits() call slow.

This patch adds a new set of recursive list apis that gives opportunity to the fs implementations to optimize. The behavior remains the same for other implementations (that is a default implementation is provided for other fs so they don't have to implement anything new). However for objectstore based fs implementations it provides a simple change to include recursive flag as true (as shown in the patch) to improve listing performance.