Refactor ProxyHttpEventArgsBase to move throttled requests to pluginData

[waldekmastykarz]

As we've introduced pluginData in ProxyEngine in #506, let's see if we can refactor ProxyHttpEventArgsBase and move throttled requests from its constructor into pluginData. Throttling belongs to plugins and shouldn't be explicitly exposed in the engine or event args.

[waldekmastykarz]

Moving information about the throttled requests to pluginData wouldn't be natural. Right now, plugin data is tied to session (single request+response). On the other hand, throttled requests apply as long as proxy is running. As such, it would be clearer to:

1. Rename pluginData to sessionData, to clearly indicate that the data is tied to a single session
2. Add globalData (or choose a more suitable name), for storing data that span multiple sessions.

Thoughts @garrytrinder?

[garrytrinder]

I agree with the approach.

In terms of naming, in the HTTP spec a request\response is referred to as an exchange, see https://www.rfc-editor.org/rfc/rfc2616#section-1.4

How about we use exchangeData, to store data for request/response exchanges and sessionData which refers to the Proxy session itself.

[waldekmastykarz]

To be honest, without knowing the spec, if I just looked at exchangeData, I'd have no idea what it's referring to. So I suggest we consider a different name. As for sessionData, Titanium uses it for a request session, so I suggest we align with it, because if we use it for proxy session it'll be confusing for others who don't have this background.

[garrytrinder]

Let's go with your original suggestions then, sessionData and globalData.