sim: have a curTick per eventq

This patch adds a _curTick variable to an eventq. This variable is updated
whenever an event is serviced in function serviceOne(), or all events upto
a particular time are processed in function serviceEvents(). This change
helps when there are eventqs that do not make use of curTick for scheduling
events.

src/mem/packet_queue.hh

@@ -56,8 +56,8 @@
 #include <list>
 
 #include "mem/port.hh"
-#include "sim/eventq.hh"
 #include "sim/drain.hh"
+#include "sim/eventq_impl.hh"
 
 /**
  * A packet queue is a class that holds deferred packets and later