Enhance HttpFilters to provide deeper insight into what's happening on the Connection level 

Hello,

I'm working with LittleProxy on a Proxy that is able to filter traffic and measure how long things took to happen at an HttpRequest/HttpResponse level.

Initially I was aiming to hook into the ActivityTracker, but then realised that a great design choice made in LittleProxy, was to associate `HttpFilters:HttpRequest=1:1`.

So, I started digging into the `ProxyToServerConnection` and discovered that it was not too difficult to keep the `HttpFilters` informed of "what was going on".

Let me know what do you think.
I'm keen to discuss improvements, corrections and refactoring as needed.

Ivan