Reduce number of calls to updateReadAcls

updateReadAcls is called on save().

But save() is called internaly for some operations like copy/move/checkin that require the DB to be fully up to date.

=> in this cases, the updateReadAcls is not really needed.

Moving the updateReadAcls from flush() to flushAndScheduleWork

=> this should help for example when using the Workflow engine (that does copy and acls changes)