utilizing mongodb TTL feature

[smartekIT]

It's faster and easier to rely on mongodb TTL feature to clean the old finished jobs after 30 days. This update is changing the format how we save the completeTime value in the database of ScriptTask. Also it adds a new index to the database to activate the TTL feature to clean finished jobs older than 30 days. Because of this index addition it will cause re-indexing of the ScriptTask db when update (no data lose at all, but it's a good thing to backup database before updating).