Netx research and refactoring in Sprint 12

[bassosimone]

Following up from https://github.com/ooni/probe-engine/issues/509. The reference PR is https://github.com/ooni/probe-engine/pull/529.

• [x] netx/dialer: optionally save bytes sent recv of every conn but don't make it dependent on the SaverDialer, rather introduce a specific Dialer for this distinct use case
• [x] netx/httptransport: make sure we roll out a robust solution for using http/httptrace because we want to make sure we don't rely on tricky/fragile behaviour.
• [x] implement specific saver for every purpose
• [x] netx/httptransport: integrate code to save body snapshots.
• [x] implement internal/archival and deprecate internal/oonidatamodel
• [x] implement experiment/urlgetter with optional psiphon tunnelling

This refactoring is functional to measure performance metrics. In turn, by doing that we will be able to measure HTTP fetching performance metrics for Psiphon. This is also a precondition to rewrite Web Connectivity in Go.

[bassosimone]

Created follow-up issue https://github.com/ooni/probe-engine/issues/576. Lots of work done in this Sprint, mainly to support metrics and Psiphon. We can close this issue now.