netx: MeasurementRoot: add RoundTrip hook

[bassosimone]

I recently gave an internal presentation to @ooni where I said that certain actions are not easy because we do not have a way to override the RoundTrip on a per request basis. Yet, if we add a RoundTrip hook to MeasurementRoot, that becomes easy to do. This can be useful to implement the ooni/netx#127 use case, for example, without changing the core implementation. Another use case is that it could allow us to use a different TLS implementation (see #229).

[bassosimone]

I spent some time experimenting with this concept in Sprint 9. I concluded that this is not desirable and that we should specifically see to reduce the amount of context usage to the minimum.

My conclusions are summarized at #359.