Taking small steps to reorganise this.

First step is to increase the safety of calling start and stop multiple times

I have seen recordings with a surprising number of apparently duplicate custom pageview events

We always register a pageview callback when starting, instead of registering one when we don't have one

So, this:

• changes _addCaptureHook to return the unsubscribe function
• calls the unsubscribe when stopping
• and only adds a listener if we don't already have one