Small changes to the segment.io event handler

Fix a couple of outstanding issues discovered during end-to-end testing:

1) The segment.io client libraries don't always set the channel field properly, allow it to be overridden.
2) Parse out the userAgent from the segment.io event and put in the normal location it is found in tracking log events.
3) The userId provided in the segment.io event is not necessarily an integer, so ensure we cast it to one before saving it out to the tracking logs.

Reviewers: @rocha, @nasthagiri
FYI: @brianhw, @stroilova, @gwprice
I have not. Last I heard they had fixed the flushing issue. Not sure if they fixed it by downgrading or changing something else.

If, in fact, they downgraded - then we do need to communicate to them the necessary field they have to set.

@stroilova - do you know the details here?