fix #842 push_mixer should exclude itself from push candidates

it fixes #842

`push_communication_impl::update_members()` should exclude `my_id_` from `servers_` vector.
In context of `communication`, server `IP:port` of itself does not make sense. (if you want to connect itself, use `my_id_`)
So `my_id_` should be excluded from `servers_` list directly.