Add a way to limit downloads bandwidth

[ksuprynowicz]

Together with #967 limiting asset download bandwidths (both http and asset server) would help users with slower internet connections. Currently upon joining the domain downloads start and cause a connect/disconnect loop that makes using game on those connections impossible. It would be worth investigating issue 967 first to see if fixing it will fix connection issues.

[daleglass]

Bandwidth limitation is likely not to be the right fix, it's much more likely there's something not quite right in the networking code. Perhaps we need better timeout handling, or better handling of packet loss