Add a "refresh ID" to key shares

[fjarri]

After a KeyRefresh/Auxiliary protocol we get a keyshare with the same verifying key, which makes it easy to confuse with an obsolete keyshare (and, for example, start a signing protocol with one). We need some kind of a publicly accessible "refresh ID" which will be updated after each refresh. Can the rho value generated during the protocol be used for that? Or we can just hash PublicAux.