WL#3591: NDB API set_name documentation and enhancements

There is an NDB API call to the set_name() method for the MGM handle that is
part of connection handle. This call works, but is not documented, it should be.

Secondly, this "name" doesn't seem to be used for anything else but for an entry
in the logfile (this is in the documentation for the set_name() function for the
MGM handle). For applications that use the NDB API, and not only MySQL Nodes,
this name would be a very worthwhile addition to be able to discriminate between
the different types of servers connected to the Cluster from the management consult.

Document:
void  Ndb_cluster_connection::set_name(const char *)

Add code to make sure that the node "name" is retained on the management server
and propagated to the MGM API (this might already be in place, but it needs to
be checked).

Add to ndb_mgm_node_state
char *name - Name of the connected node.

Probably this should also be visible in the ndb_mgm client, but as this might
break existing scripts and the name also is only useful for customers using the
NDB API, showing the name should maybe be optional.