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/stream.h b/include/types/stream.h
index b83f655..be28568 100644
--- a/include/types/stream.h
+++ b/include/types/stream.h
@@ -139,7 +139,7 @@
 	int store_count;
 	/* 4 unused bytes here */
 
-	struct stkctr stkctr[MAX_SESS_STKCTR];  /* stick counters */
+	struct stkctr stkctr[MAX_SESS_STKCTR];  /* content-aware stick counters */
 
 	char **req_cap;                         /* array of captures from the request (may be NULL) */
 	char **res_cap;                         /* array of captures from the response (may be NULL) */