proxy: add `toServer` invocation

    now, we can skip the route calculating when initiating an rpc with
    specifying a certain remote server. for example:

    app.rpc.<RemoteServerType>.<Remote>.<Method>.toServer('server-id',
        ...);

    and, use `*` to be as wildcard for all the servers that have the
    same server type.
Merge branch 'add-toServerId'