Change sorting of tasks in FileDownload agent

[nikmagini]

Currently FileDownload sorts tasks by time_assign asc then rank asc. Meaning that a low priority task will always win over a high priority task created only a few minutes later.

Suggestion by transfer team is to use a more clever sorting function so that lower rank (higher priority) tasks get a priority boost for some time.

[nikmagini]

This would also require the FileIssue agent to assign rank to new tasks looking at the rank of all preexisting tasks, instead of doing it independently on each cycle. To be estimated the impact on FileIssue performance.