Improve performance of retrieving last update times for events

See 75195eefcca4cb3961986c90c8e837da02f30134 and
https://gitlab.com/gitlab-org/gitlab-ce/issues/12415#note_3387317
for the juicy details.

Related issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/12415

See merge request !2613