[escher] Add an InvocationTimer subclass

So that we can build invocation-less timers

Change-Id: I412fc8ad040156340caab295719fb83ed25b8b51

escher/src/invocation_timer.cpp

@@ -0,0 +1,11 @@
+#include <escher/invocation_timer.h>
+
+InvocationTimer::InvocationTimer(Invocation invocation, uint32_t period) :
+  Timer(period),
+  m_invocation(invocation)
+{
+}
+
+void InvocationTimer::fire() {
+  m_invocation.perform(this);
+}