Fix queue time calculation

[asgrim]

Fixes #163

[dlanderson]

I don't think this will work; I'd like to make this more robust. Take a look at how we do it in our Python agent. It should be easy to port over. A few things to note:

1. The headers can be any of X-Queue-Start X-Request-Start X-QUEUE-START X-REQUEST-START x-queue-start x-request-start
2. The value of the header can start with t=, but not always.
3. We can detect ambiguous values by anchoring a start date and comparing the values.

[dlanderson]

See comment #169 (comment)

[asgrim]

@dlanderson

1. The header matching is already case insensitive in PHP, so any of those values will work fine (the casing is applied consistently in \Scoutapm\Helper\FetchRequestHeaders::fromArray so the header names are deterministic here)
2. Good catch, I missed that
3. Interesting, all the docs I could find (which wasn't much!) on these headers was that they were always in milliseconds. I'll take a look into this 👍