This is a consolidation effort, avoiding usage
of naked strings in codebase. Whenever possible
use constants which can be repurposed elsewhere.
This also fixes `goconst ./...` reported issues.
- Reads and writes of uploads.json in XL now uses quorum for
newMultipart, completeMultipart and abortMultipart operations.
- Each disk's `uploads.json` file is read and updated independently for
adding or removing an upload id from the file. Quorum is used to
decide if the high-level operation actually succeeded.
- Refactor FS code to simplify the flow, and fix a bug while reading
uploads.json.
