Add support for use of snapshot diff for external table replication.
DistCp: Backward compatibility: Distcp fails from Hadoop 3 to Hadoop 2 for snapshotdiff
DistCp: Reduce memory usage on copying huge directories
DistCp: Use Iterator for listing target directory as well
Create a single copy task for external tables within default DB location
Allow creating single copy tasks for configured paths during external table replication
Allow custom configs for database level paths in external table replication
GitHub Pull Request #2043