Support import endpoint within track method for older events

Support using the import endpoint instead of the track endpoint when an event has a time that's older than 5 days in the past.

There's lots of ways to handle this but these changes were done in a manner to allow existing users to seamlessly continue using the package as they are, while allowing those that want the extra functionality to take advantage of it. This involves fitting the added logic into the existing code structure instead of rewriting a lot to make things ideal but then breaking the existing API in the process. So - some potential awkwardness for the sake of API compatibility.