[MEDIUM] stick-tables: add a reference counter to each entry

We'll soon have to maintain links from sessions to entries, so let's
add a refcount in entries to avoid purging them if it's not null.
diff --git a/include/types/stick_table.h b/include/types/stick_table.h
index 843f5d7..25bfbd8 100644
--- a/include/types/stick_table.h
+++ b/include/types/stick_table.h
@@ -75,6 +75,7 @@
  */
 struct stksess {
 	unsigned int expire;      /* session expiration date */
+	unsigned int ref_cnt;     /* reference count, can only purge when zero */
 	struct eb32_node exp;     /* ebtree node used to hold the session in expiration tree */
 	struct ebmb_node key;     /* ebtree node used to hold the session in table */
 	/* WARNING! do not put anything after <keys>, it's used by the key */