gitserver: limit internal cloning to 1 Gbps

Created by: keegancsmith

zoekt-sourcegraph-indexserver currently limits the rate at which a git archive is fetched to 1 Gbps. This is to prevent us overloading the network. Before we had the limit we would cause the docker networking stack to crash. Since zoekt is moving to using our internal clone endpoint, we want to keep the limiting behaviour. Instead of limiting on the client side, we limit on the server side. This is done since introducing limiting in git would require extra tools to be installed.

The comment and limit is the same we have currently for the archive based approach.