Rolling update doesn't work properly because Storage REST API has
a new API WriteAll() but without API version number increase.
Also be sure to return 404 for unknown http paths.
xl.json is the source of truth for all erasure
coded objects, without which we won't be able to
read the objects properly. This PR enables sync
mode for writing `xl.json` such all writes go hit
the disk and are persistent under situations such
as abrupt power failures on servers running Minio.
