mtree: internal prefix max size set to 64

- allow for larger prefix values to be used (e.g., case of uuid, ipv6)
- db corresponding field still set to varchar(32)

(cherry picked from commit 47086a4ee0a6ee6a766d7591e91e5663acf31562)

modules/mtree/mtree.h

@@ -59,7 +59,7 @@ typedef struct _mt_node
     struct _mt_node *child;
 } mt_node_t;
 
-#define MT_MAX_DEPTH	32
+#define MT_MAX_DEPTH	64
 
 #define MT_NODE_SIZE	mt_char_list.len