Variable chunk send rate based on available bandwidth

[dktapps]

Description

This requires pmmp/RakLib#32.

Currently, we send chunks to clients at a fixed rate of 4 per tick.

While this works mostly fine, we have to be mindful of the lowest-end connections around when adjusting it. This means that the send rate of terrain is dictated by the worst connections available.

This issue proposes turning the setting into into a max sending rate. We would need a way to know how much bandwidth clients have available, to allow throttling the send rate for specific clients with poor connections, while allowing fast connections to download the terrain as fast as possible.

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