HDFS-13616 added a new API for batch listing of multiple directories, but
it isn't yet ready for tagging as stable & doesn't suit object stores.
- the new API is pulled into a new interface marked unstable;
- new classes (PartialListing) also tagged unstable.
- Define a new path capability.
HDFS will implement, but not filter/HarFS; it is an HDFS exclusive implementation for now.