[feature] Issue: #45 Support for INVISIBLE indexes

This feature is ported from upstream 8.0.

As 8.0 introduces Data Dictionary which substitute metadata files
in server layer used in previous version(such as frm) , so there are
some subtle differences.
1. we store index visibility info in frm
2. we use the HA_SORT_ALLOWS_SAME flag, which was not stored in frm
   before, in frm to indicate HA_INVISIBLE_KEY, so that we don't need
   to expand frm format, and compatibility is held.

include/my_base.h

@@ -260,6 +260,7 @@ enum ha_base_keytype {
 #define HA_SPATIAL		1024    /* For spatial search */
 #define HA_NULL_ARE_EQUAL	2048	/* NULL in key are cmp as equal */
 #define HA_GENERATED_KEY	8192	/* Automaticly generated key */
+#define HA_INVISIBLE_KEY        (1<<30) /* This key is visible */
 #define HA_CLUSTERING           (1<<31) /* TokuDB CLUSTERING key */
 
         /* The combination of the above can be used for key type comparison. */