Send turnstile event when telemetry is paused

[friedbunny]

The turnstile event is used solely to count monthly active users and should always be sent, regardless of whether the user has opted themselves out of telemetry.

The turnstile wasn't being turned if telemetry was paused, because _session is invalided and nil'd in pauseMetricsCollection. This change instantiates a temporary NSURLSession for the turnstile event.

@1ec5 I could use your advice on my use of strongSelf.session here — before, we were straight-up calling _session, but inside of a background async block.

[1ec5]

Rather than checking for the existence of MGLMapboxEvents.session, I think it’d be better to check for paused state, or better yet, for an indication that this is a turnstile event. If we’re getting in here without a session for anything other than a turnstile event, that’s an error on our part, so the original code attempts to fail gracefully.

Additionally, could we keep this synchronous, turnstile-specific session a local variable, to avoid confusion about what kind of session is being stored in the ivar?

[friedbunny]

Making this more specific to the turnstile sounds good, forget why I didn't go with isPaused here but will look at casing this to the turnstile only.

👍 on using a standalone NSURLSession here.

[friedbunny]

Squashed and rebased, will merge when the tests go green.