Simplify updating queue entries

The erase/insert pattern can be replaced by a single method that
updates an existing entry's timer, or inserting if it doesn't exist.

This has much less impact than the libtorrent change it's based on,
but might as well be consistent.

rak/priority_queue.h

@@ -74,6 +74,10 @@ public:
     base_type::pop_back();
   }
 
+  void update() {
+    std::make_heap(begin(), end(), m_compare);
+  }
+
   void push(const value_type& value) {
     base_type::push_back(value);
     std::push_heap(begin(), end(), m_compare);
@@ -95,12 +99,8 @@ public:
     return true;
   }
 
-  // Removes 'itr' from the queue. This assumes 'itr' has been
-  // modified such that it has a higher priority than any other
-  // element in the queue.
+  // Removes 'itr' from the queue.
   void erase(iterator itr) {
-//     std::push_heap(begin(), ++itr, m_compare);
-//     pop();
     base_type::erase(itr);
     std::make_heap(begin(), end(), m_compare);
   }