session: measure dispatch period only if under load

Also reset pacer on tcp retransmit timeout

Change-Id: I5a9edee4c00d1d169248d79587a9b10437c2bd87
Signed-off-by: Florin Coras <fcoras@cisco.com>
diff --git a/src/vnet/session/session.h b/src/vnet/session/session.h
index 131652a..3bdf6c9 100644
--- a/src/vnet/session/session.h
+++ b/src/vnet/session/session.h
@@ -229,6 +229,8 @@
   /** Peekers rw lock */
   clib_rwlock_t peekers_rw_locks;
 
+  u32 last_tx_packets;
+
 } session_manager_worker_t;
 
 struct _session_manager_main