MINOR: session: add stick counters to the struct session

The stick counters in the session will be used for everything not related
to contents, hence the connections / concurrent sessions / etc. They will
be usable by "tcp-request connection" rules even without a stream. For now
they're just allocated and initialized.
diff --git a/include/types/session.h b/include/types/session.h
index e4e393b..45d2df1 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -41,6 +41,7 @@
 	enum obj_type *origin;          /* the connection / applet which initiated this session */
 	struct timeval accept_date;     /* date of the session's accept() in user date */
 	struct timeval tv_accept;       /* date of the session's accept() in internal date (monotonic) */
+	struct stkctr stkctr[MAX_SESS_STKCTR];  /* stick counters for tcp-connection */
 };
 
 #endif /* _TYPES_SESSION_H */