Improve the reliability of the Cluster refresh process for an edge case.

Export HashSlots constant to make it nicer to write the Cluster.LayoutRefresh signature.

Fix closing/releasing of connections used in Cluster.EachNode.

Add Cluster.EachNode to call a function with a connection for each known node in the cluster; add optional Cluster function field BgError to receive notification of errors happening in background topology refreshes and on closing of RetryConn after following a redirection to a new connection; add optional Cluster function field LayoutRefresh to receive the old and new cluster slot mappings to server address(es); and more, see the full changelog in the README.