AthenaConfiguration: Add a CA flag to control loop manager's EventPrintoutInterval

By default all loop managers report the beginning/ending of each event and updates the event-loop heartbeat file. The former clutters the log file until we hit the default 500 messages limit but especially the latter stay on and can be problematic for jobs that processes events very quickly, e.g., see ATEAM-971. This MR adds a new CA flag that allows us to configure this.

I guess we can change the default here as well. Any choice will be arbitrary but maybe something like 100 events or so would be more sensible than 1, which is what we already do in some skeletons, e.g., see DerivationSkeleton.py. If we do this, we should make sure we don't break any unit tests that do log file comparisons, though.

Let me tag a few people here in case they have any opinion: @jcatmore, @jchapman, @akraszna, @fwinkl.