[escher] Add an InvocationTimer subclass

So that we can build invocation-less timers

Change-Id: I412fc8ad040156340caab295719fb83ed25b8b51

escher/src/timer.cpp

@@ -1,7 +1,6 @@
 #include <escher/timer.h>
 
-Timer::Timer(Invocation invocation, uint32_t period) :
-  m_invocation(invocation),
+Timer::Timer(uint32_t period) :
   m_period(period),
   m_numberOfTicksBeforeFire(period)
 {
@@ -18,7 +17,3 @@ void Timer::tick() {
 void Timer::reset() {
   m_numberOfTicksBeforeFire = m_period;
 }
-
-void Timer::fire() {
-  m_invocation.perform(this);
-}