AthenaPoolCnvSvc: Add a default compression setting for standard EVNT files
This MR adds a default compression setting for the standard EVNT files. In a standard nonallhad ttbar sample, the EVNT event size seems to be about 60 KiB. Based on this, a reasonable AutoFlush could be around 500 events. So, let's go with that.
Related to ATEAM-1014