CLEANUP: connection: use pool_zalloc() in conn_alloc_hash_node()

This one used to alloc then zero the area, let's have the allocator do it.
diff --git a/include/haproxy/connection.h b/include/haproxy/connection.h
index 7370263..4f03aef 100644
--- a/include/haproxy/connection.h
+++ b/include/haproxy/connection.h
@@ -376,12 +376,10 @@
 {
 	struct conn_hash_node *hash_node = NULL;
 
-	hash_node = pool_alloc(pool_head_conn_hash_node);
+	hash_node = pool_zalloc(pool_head_conn_hash_node);
 	if (unlikely(!hash_node))
 		return NULL;
 
-	memset(&hash_node->node, 0, sizeof(hash_node->node));
-	hash_node->hash = 0;
 	hash_node->conn = conn;
 
 	return hash_node;