reverse proxy: adjust dst when reading flows, fix #346

As discussed earlier today:

Assuming someone starts mitmproxy in reverse proxy mode with recorded flows, e.g.
`mitmproxy -r recorded_flows -R http://127.0.0.1:80`, we now set the destination for each flow to the reverse proxy. This allows to record flows and replay them to a different server later on.

Your earlier comment <i>"there's always been a number of different places we could source the host from in the display."</i> doesn't apply anymore - it's always request.host now (#337). As we don't want to break that up again, we already modify the target when loading the flow.