DRILL-4720: Fix SchemaPartitionExplorer.getSubPartitions method implementations to return only Drill file system directories
1. Added file system util helper classes to standardize list directory and file statuses usage in Drill with appropriate unit tests.
2. Fixed SchemaPartitionExplorer.getSubPartitions method implementations to return only directories that can be partitions according to Drill file system rules
(excluded all files and directories that start with dot or underscore).
3. Added unit test for directory explorers UDFs with and without metadata cache presence.
4. Minor refactoring.
closes #864
