getBranchTips must consider official series tagging on "since" handling

The getBranchTips call in the API returns a list of branches that have changed after a given timestamp provided via the "since" parameter. This is useful to reduce the load in Launchpad by scanning only recent changes in the distribution rather than the whole history.

Unfortunately, though, there's a situation that still forces the full scan to be done and renders "since" not useful: the third parameter in the response to this query is a list of series where the given branch is tagged as official for, and changes to this list after the provided timestamp will not cause the entry to show up again in the response.

For us to be able to use the "since" parameter and save Launchpad some CPU, we need such changes to be considered in the response as well.

Francis pointed out that the marks already have last-updated timestamps, which means the change should be relatively simple for someone well versed in the server-side implementation of the logic.