BibSched: adjust runtimes when using sequence-id

• Whithin a set of tasks having the same sequence-id, when, according to its runtime value, a task is scheduled to be executed after another task which has higher task_id, the other task's runtime is adjusted accordingly to be executed after the earlier task.

• When a task is postponed because it happens to be executed outside of its time limit, all the other tasks in the same sequence id, with an earlier runtime but a later task_id are also postponed automatically. (closes #1145)