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/sim/core.cc

@@ -35,11 +35,10 @@
 #include "base/callback.hh"
 #include "base/output.hh"
 #include "sim/core.hh"
+#include "sim/eventq.hh"
 
 using namespace std;
 
-Tick _curTick = 0;
-
 namespace SimClock {
 /// The simulated frequency of curTick(). (In ticks per second)
 Tick Frequency;