This patch modifies multipart upload related functions as below
* New multipart upload call creates file
EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.uploadid
* Put object part call creates file
EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.PART_NUMBER.MD5SUM_STRING
* Abort multipart call removes all files matching
EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.*
* Complete multipart call does
1. creates a staging file
EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.complete.TEMP_NAME
then renames to
EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.complete
2. rename staging file
EXPORT_DIR/.minio/BUCKET/PATH/TO/OBJECT/UPLOAD_ID.complete
to EXPORT_DIR/BUCKET/PATH/TO/OBJECT
