Calling /minio/prometheuses/metrics calls xlSets.StorageInfo() which creates a new
storage REST client and closes it. However, currently, closing does nothing
to the underlying opened http client.
This commit introduces a closing behavior by calling CloseIdleConnections
provided by http.Transport upon the initialization of this latter.
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.
