Discard gen_server replies after a timeout in GM

[dcorbacho]

GM will crash if the call to its neighbour timeouts, but the answer arrives later on through handle_info as a tagged message.

Any unmatched message should be discarded in the handle_info of this gen_server.

** Reason for termination ==
** {function_clause,
       [{gm,handle_info,
            [{#Ref<0.0.4718594.221880>,
              {ok,{gm_group,
                      {resource,
                          <<"anonymous">>,
                          queue,
                          <<"anonymous">>},
                      4,
                      [{0,<7938.17798.5>},{3,<0.18371.9>}]}}},