mem: Redesign the stack distance calculator as a probe

This changeset removes the stack distance calculator hooks from the
CommMonitor class and implements a stack distance calculator as a
memory system probe instead. The probe can be hooked up to any
component that exports probe points of the type ProbePoints::Packet.

src/mem/SConscript

@@ -44,7 +44,6 @@ SimObject('ExternalMaster.py')
 SimObject('ExternalSlave.py')
 SimObject('MemObject.py')
 SimObject('SimpleMemory.py')
-SimObject('StackDistCalc.py')
 SimObject('XBar.py')
 
 Source('abstract_mem.cc')