Change to HttpSM history

[Alan Carroll]

I was looking at commit for e634659783bcadbedeb1bca411d36640d18b3b59 PR
2272. This moves the history mechanism to separate file. It also changes
history from a circular buffer to a linear buffer. This means the events
stored in it are the first events for the HttpSM, not necessarily the most
recent ones. I consider this a rather significant change but I see no
discussion of that in the PR and I don't remember it on the mailing list.
Did I miss it? I much prefer the circular buffer style.