BibSched: scheduling algorithm improvements

• Correctly rebuild tasks view when some priorities or runtimes are adjusted in a sequenceid set.

• Clearly consider any other task in a sequenceid set when deciding whether to start a given task. (closes #1281)

• Optimizes waking-up decisions by not waking up tasks when there are other tasks about to go to sleep, since these usually means there is some higher priority task incoming, and the current task that could be waked up would steal their slot (while immediately be put to sleep on the next bibsched iteration).