Production()/Development() is not thread-safe (and more generally ClientManager will have inconsistent results if switching between Development/Production)

[froodian]

1. go version go1.8.3
2. darwin/amd64
3. Called client.Production() / client.Development() on clients return from ClientManager.Get
4. Expected no data race
5. Had "data race detected" from a test, between the line in my code calling client.Production() and apns2/client.go:156 (url := fmt.Sprintf("%v/3/device/%v", c.Host, n.DeviceToken))

[froodian]

I solved this by just keeping a ClientManager full of production clients and a ClientManager full of development clients, I feel like that's a fine solution and removes the ambiguity if another thread is sending on the client when you try to switch gateways, so I feel like that's a good answer to this, closing. Sorry about the noise.