mem: Move trace functionality from the CommMonitor to a probe

This changeset moves the access trace functionality from the
CommMonitor into a separate probe. The probe can be hooked up to any
component that exports probe points of the type ProbePoints::Packet.

This patch moves the dependency on Google's Protocol Buffers library
from the CommMonitor to the MemTraceProbe, which means that the
CommMonitor (including stack distance profiling) no long depends on
it.

src/mem/SConscript

@@ -30,11 +30,8 @@
 
 Import('*')
 
-# Only build the communication if we have support for protobuf as the
-# tracing relies on it
-if env['HAVE_PROTOBUF']:
-    SimObject('CommMonitor.py')
-    Source('comm_monitor.cc')
+SimObject('CommMonitor.py')
+Source('comm_monitor.cc')
 
 SimObject('AbstractMemory.py')
 SimObject('AddrMapper.py')