Add feature: Update members.

First create an `UpdateMember` command from a `Member`.

    update_member = UpdateMember(member)

The fields of `update_member` can be modified.

    update_member.route = 'dummy route'
    update_member.route_redirect = 'dummy route redirect'
    update_member.load_factor = 1
    update_member.lb_set = 1
    update_member.ignore_errors = True
    update_member.drainig_mode = True
    update_member.enabled = True
    update_member.hot_standby = True

Use the `BalancerManager` to execute the update.

    balancer_manager.send_command(update_member)