FileSystemPlugin refactoring: avoid sharing DrillFileSystem across schemas

Currently we create one DrillFileSystem (an extension of hadoop FileSystem) instance and share it across all Workspaces created for all queries, FormatPlugins and FormatMatcher. Remove the shared DrillFileSystem instead share the DrillFileSystem configuration and create a DrillFileSystem in each Schema (WorkspaceSchema) using the current user credentials in Schema. The same DrillFileSystem instances to passed to FormatPlugins and FormatMatchers whenever Schemas need to access the file system.