Fix config phase packets getting lost

[valaphee]

When the config-phase is initiated externally, packets are getting lost, as there is no connectionInFlight.

This is most noticeable with PluginMessagePacket as some client-only mods initiate their state in the config phase. The servers request is receipt, but the client has no way to acknowledge the pm.

One common scenario where it would occur is when you chain multiple Velocity servers. But it should be the expected behavior nonetheless.