#8931 - Add bandwidth throttling

The basic mechanism for bandwidth throttling is there, here are some comments regarding the implementation:

• The HTTPConnectionPool creates one factory for each connection, so I had to limit the number of connections of the pool to 1 when throttling.
• I have not added tests as the twisted code base has tests for the throttling mechanism.
• We will need to add a way to configure the limits when we actually start using this feature.