webconnectivity LTE collects additional metrics to facilitate throttling measurements

[bassosimone]

This issue is a child of #1296. The general idea is to take into account ooni/probe-cli#684 and ooni/probe-cli#959 to ensure we collect download speed metrics in a more robust way. While there, we should also review other micro-dataformats and see if we're collecting enough information.

The plan is to integrate this functionality in webconnectivity LTE (see ooni/probe#2237).

[bassosimone]

We implemented the core functionality required to measure throttling in ooni/probe-cli#1166. The functionality currently only works with Web Connectivity LTE. We need to switch more tests to use the measurexlite measurement engine (and the DSL on top of it) to take advantage of this functionality. Additionally, we will not be collecting data for this functionality until we complete ooni/probe#2555. In any case, as far as merging code that does the job(TM) is concerned, we're good. So closing the issue.