Convert iomux to use an UCoreClock

include/ucore/iomux_impl.h

@@ -9,10 +9,12 @@ struct IOMux {
     /* TImer instance */
     struct UCTimers timers;
 
-    /* Current time. Updated before calling run_impl, and updated
-     * by iomux_timers_run
+    /**
+     * Clock used for driving timers, and
+     * providing timeouts for the mux
      */
-    struct timeval now;
+    struct UCoreClock *uclock;
+
 
     /* Used by mux implementations to hold their own data/instance */
     void *instance;