ConnStats: Expose num_packets_unprocessed

Not sure it's the best place to put, but we don't have packet analysis stats
bif and also num_packets is already there, so seems reasonable to put the
num_packets_unprocessed into

src/session/Manager.h

@@ -50,6 +50,7 @@ struct Stats {
     size_t num_fragments;
     size_t max_fragments;
     uint64_t num_packets;
+    uint64_t num_packets_unprocessed;
 };
 
 class Manager final {