[kernel] Change to one-shot timer scheduler

Instead of many timer interrupts and decrementing a process' remaining
quanta, change to setting a single timer for when a process should be
preempted. If it uses its whole timeslice, demote it. If it uses less
than half before blocking, promote it. Determine timeslice based on
priority as well.

This change also required changing the apic timer interface to be purely
interval (in microseconds) based instead of its previous interval/tick
hybrid.

src/kernel/interrupts.cpp

@@ -210,7 +210,7 @@ isr_handler(cpu_state *regs)
 		break;
 
 	case isr::isrTimer:
-		scheduler::get().tick();
+		scheduler::get().schedule();
 		break;
 
 	case isr::isrLINT0: