First version of HTTP bandwidth throttling.

Adds throttling based on on average bandwidth usage per HTTP service.
Since only HTTP textures are using this, they are still starved by other
services like inventory and mesh dowloads. Also, it will be needed to
move the maximum number of connections per service the to the PerService
class, and dynamically tune them: reducing the number of connections is
the first thing to do when using too much bandwidth.

I also added a graph for HTTP texture bandwidth to the stats floater.
For some reason the average bandwidth (over 1 second) look almost like
scattered noise... weird for something that is averaged...