mem: Add tracing support in the communication monitor

This patch adds packet tracing to the communication monitor using a
protobuf as the mechanism for creating the trace.

If no file is specified, then the tracing is disabled. If a file is
specified, then for every packet that is successfully sent, a protobuf
message is serialized to the file.

src/proto/SConscript

@@ -41,4 +41,5 @@ Import('*')
 
 # Only build if we have protobuf support
 if env['HAVE_PROTOBUF']:
+    ProtoBuf('packet.proto')
     Source('protoio.cc')