Werk #8552: Notification spooling: Fixed "Corrupted spoolfile" in case spool files are created

Fixed a race condition between the producing side (the notification module which creates the spool file) and the consuming side (the mknotifyd which processes the spool files) that made the mknotifyd process incompletely written files.

This could cause "corrupted spoolfile" error messages in the var/log/mknotifyd.log and resulted in spool files being moved to the directory var/check_mk/notify/corrupted. The notifications are not sent out in this situation.

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