session/tcp: support tx flush mark

For tcp this means that the last enqueued data goes out with a psh bit
set.

Change-Id: I29d357ecae6f02e748b59a7b799150ec73d14ba2
Signed-off-by: Florin Coras <fcoras@cisco.com>
diff --git a/src/vnet/session/transport_interface.h b/src/vnet/session/transport_interface.h
index 3bfed41..10579c4 100644
--- a/src/vnet/session/transport_interface.h
+++ b/src/vnet/session/transport_interface.h
@@ -61,6 +61,7 @@
   u32 (*send_space) (transport_connection_t * tc);
   u32 (*tx_fifo_offset) (transport_connection_t * tc);
   void (*update_time) (f64 time_now, u8 thread_index);
+  void (*flush_data) (transport_connection_t *tconn);
 
   /*
    * Connection retrieval