sessions_between_states metric

Similar to sessions_until_state metric, but allows you to specify the state where you start counting sessions.  This will allow you to evaluate specific sections of an app's performance in terms of session count.
I'm not sure if this should be `0` or `1`. `0` makes sense if you think that there were no new sessions between the two states, so the value is `0`, `1` makes sense if you think that in order to get through those states, you had to be in a sessions. I'm open to suggestion here.