Modify thread_set_priority

"When the donations are released, the thread's priority becomes the one
set through the function call."
Merge remote-tracking branch 'origin/master' into borui

Conflicts:
	src/threads/thread.c
Merge remote-tracking branch 'origin/master' into borui

Conflicts:
	src/threads/DESIGNDOC
	src/threads/synch.c
	src/threads/thread.c
Clear commented codes in timer.c
Update the purpose of wake_time
Refine DESIGNDOC - ALARM CLOCK
Refine DESIGNDOC PRI
some update
old_priority?
old_priority? cont.
contt
Revert "contt"

This reverts commit 097c4bbcfd5910c481688b521cde602a383f0ee0.
Revert "old_priority? cont."

This reverts commit 21a74273701737c85260d4bd7fc86ef0653f3eab.
Revert "old_priority?"

This reverts commit b25b85d00ac968dbcbb31a2c1224d2af4c3be63f.