DRILL-4257: Fix StoragePluginRegistry clean-up behavior and misc clean up.
- Create the storage plugin registry using a Drill configuration parameter to be able to replace the registry implementation.
- Change StoragePluginRegistry into an interface and move the implementation to an impl class.
- Write documentation for StoragePluginRegistry.
- Make StoragePluginRegistry and StoragePluginMap AutoCloseable and ensure that Drillbit closes registry.
- Misc Drillbit code reorganization so that fields are at top of class (and static methods at bottom).
- Update DrillConfig to support reflection-based creation of storage plugin objects
- Remove final from DrillConfig so that application developers can extend config object.
This closes #321.
