Require TLS 1.2 minimum.

This prevents a downgrade attack to the vulnerable SSLv3.  While here,
go ahead and require at least TLS 1.2 since TLS 1.0 and 1.1 have their
own set of issues and it's only a matter of time before those would
need to be completely avoided as well.

ok @davecgh

rpcserver.go

@@ -539,6 +539,7 @@ func newRPCServer(listenAddrs []string, s *server) (*rpcServer, error) {
 
 	tlsConfig := tls.Config{
 		Certificates: []tls.Certificate{keypair},
+		MinVersion:   tls.VersionTLS12,
 	}
 
 	// TODO(oga) this code is similar to that in server, should be