clearcase ucm: we arent storing the latest possible activity time, so its possible to get a lot of unnecessary data

when calling lshistory on the components in a branch, at the moment we use the "oldest of the latest times from each component" as the next index date. this was so that we could make sure that we didnt lose any data.

we can do better than this - we should use "latest time across components" and subtract the processing time. it will still potentially be getting a little bit too much data, but it will be much less than currently.