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/python/swig/event.i

@@ -33,7 +33,7 @@
 %{
 #include "base/types.hh"
 #include "python/swig/pyevent.hh"
-#include "sim/eventq.hh"
+#include "sim/eventq_impl.hh"
 #include "sim/sim_events.hh"
 #include "sim/sim_exit.hh"
 #include "sim/simulate.hh"