Allow workaround for discovery startup race

[KnicKnic]

Related to issue #257

This is not full fix, however will allow an application that is written correctly to not hit the race. What the application would have to do is create the server with no backends, and then immediately update the backends with the backend pool that they want.

This race can be hit using the rest api, or when embeding gobetween as a library.

manager.Create("foo", config.Server{
    Protocol: "tcp",
    Bind:     "127.0.0.1:199",
    Discovery: &config.DiscoveryConfig{
        Kind:                  "static",
        StaticDiscoveryConfig: &config.StaticDiscoveryConfig{StaticList: []string{}},
    },
})
manager.UpdateBackends("foo", &[]core.Backend{core.Backend{Target: core.Target{Host: "127.0.0.1", Port: "8990"}, Stats: core.BackendStats{Live: true}}})

[KnicKnic]

Builds on or https://github.com/yyyar/gobetween/pull/256/files