registering notFound handler more than once causes
gorilla mux to return error for all registered paths
greater than > 8. This might be a bug in the gorilla/mux
but we shouldn't be using it this way. NotFound handler
should be only registered once per root router.
Fixes#6915
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.
