Do best effort to download the experiments and use it in the very first session only

[karrtikr]

Only for the first session, try downloading the experiments before activating the extension.

• If the download completes under 2 seconds, we use the downloaded experiments for the first session.
• If the download fails within 2 seconds, log error and use the existing experiments.json to get the experiments.
• If the download does not complete within 2 seconds, fallback to what we were doing. That is, use the existing experiments.json to get the experiments.

Log telemetry for each of the 3 options. Activating the extension for the first time will take 2 more seconds as a consequence.

It will benefit us in case we want to change experiments mid-session, and want to make sure that the first session captures the new experiments. For instance,

In case of LS - enabled, for the users new to intellisense, we want to make sure the first experience (which could be the only experience) is Language server. Right now, what could happen is the first session will activate with Jedi, and LS will only be activated from the second session onwards.

[karrtikr]

cc/ @qubitron @luabud @brettcannon