ADL connector: change object protection when a new index file is created

[christineyost]

The purpose of this change is to fix issue #1962

• Change DataLakeFileSystemClient.createFile call to DataLakeFileSystemClient.createFileIfNotExists to prevent overwriting an existing file in writeBlobToFile. This method call will still return the DataLakeFileClient if the file already exists.
• Remove the condition to prevent the flush operation from happening on any file that already has an etag in the case where a new Index file is being created.
• Add a unit test.