[MEDIUM] stick_table: move the server ID to a generic data type

The server ID is now stored just as any other data type. It is only
allocated if needed and is manipulated just like the other ones.
diff --git a/src/stick_table.c b/src/stick_table.c
index 994fbeb..017362f 100644
--- a/src/stick_table.c
+++ b/src/stick_table.c
@@ -64,7 +64,6 @@
 static struct stksess *stksess_init(struct stktable *t, struct stksess * ts)
 {
 	memset((void *)ts - t->data_size, 0, t->data_size);
-	ts->sid = 0;
 	ts->key.node.leaf_p = NULL;
 	ts->exp.node.leaf_p = NULL;
 	return ts;
@@ -476,6 +475,7 @@
 /* Extra data types processing */
 struct stktable_data_type stktable_data_types[STKTABLE_DATA_TYPES] = {
 	[STKTABLE_DT_CONN_CUM] = { .name = "conn_cum", .data_length = stktable_data_size(conn_cum) },
+	[STKTABLE_DT_SERVER_ID] = { .name = "server_id", .data_length = stktable_data_size(server_id) },
 };
 
 /*