Kernel: Add kernel-level timer queue (heavily based on @juliusf's work)

PR #591 defines the rationale for kernel-level timers. They're most
immediately useful for TCP retransmission, but will most likely see use
in many other areas as well.

Kernel/TimerQueue.cpp

@@ -0,0 +1,72 @@
+#include <AK/Function.h>
+#include <AK/NonnullOwnPtr.h>
+#include <AK/OwnPtr.h>
+#include <Kernel/Scheduler.h>
+#include <Kernel/TimerQueue.h>
+
+static TimerQueue* s_the;
+
+TimerQueue& TimerQueue::the()
+{
+    if (!s_the)
+        s_the = new TimerQueue;
+    return *s_the;
+}
+
+u64 TimerQueue::add_timer(NonnullOwnPtr<Timer>&& timer)
+{
+    ASSERT(timer->expires > g_uptime);
+
+    timer->id = ++m_timer_id_count;
+
+    auto following_timer = m_timer_queue.find([&timer](auto& other) { return other->expires > timer->expires; });
+    if (following_timer.is_end())
+        m_timer_queue.append(move(timer));
+    else
+        m_timer_queue.insert_before(following_timer, move(timer));
+
+    update_next_timer_due();
+
+    return m_timer_id_count;
+}
+
+u64 TimerQueue::add_timer(u64 duration, TimeUnit unit, Function<void()>&& callback)
+{
+    NonnullOwnPtr timer = make<Timer>();
+    timer->expires = g_uptime + duration * unit;
+    timer->callback = move(callback);
+    return add_timer(move(timer));
+}
+
+bool TimerQueue::cancel_timer(u64 id)
+{
+    auto it = m_timer_queue.find([id](auto& timer) { return timer->id == id; });
+    if (it.is_end())
+        return false;
+    m_timer_queue.remove(it);
+    update_next_timer_due();
+    return true;
+}
+
+void TimerQueue::fire()
+{
+    if (m_timer_queue.is_empty())
+        return;
+
+    ASSERT(m_next_timer_due == m_timer_queue.first()->expires);
+
+    while (!m_timer_queue.is_empty() && g_uptime > m_timer_queue.first()->expires) {
+        auto timer = m_timer_queue.take_first();
+        timer->callback();
+    }
+
+    update_next_timer_due();
+}
+
+void TimerQueue::update_next_timer_due()
+{
+    if (m_timer_queue.is_empty())
+        m_next_timer_due = 0;
+    else
+        m_next_timer_due = m_timer_queue.first()->expires;
+}