Every 1000 log entry inserted, Piwigo performs an history summarize.
The summarize process has also been optimized: no longer used column
history.summarized (no longer need to update it, which took a lot in time),
we now save the history_id_from and history_id_to in history_summary table.
This way we know from where to start on next summarize.
For now, for a simple performance reason, we keep column history.summarized,
because removing it may take a long time on huge tables. Once we will have
automatic purge on history, it will be safer to drop this column.
