Before this commit, nodes wait indefinitely without showing any
indicate error message when a node is started with different access
and secret keys.
This PR will show '401 Unauthorized' in this case.
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.
