Added page_range to records_in_range() to improve range statistics

Prototype change:
-  virtual ha_rows records_in_range(uint inx, key_range *min_key,
-                                   key_range *max_key)
+  virtual ha_rows records_in_range(uint inx, const key_range *min_key,
+                                   const key_range *max_key,
+                                   page_range *res)

The handler can ignore the page_range parameter. In the case the handler
updates the parameter, the optimizer can deduce the following:
- If previous range's last key is on the same block as next range's first
  key
- If the current key range is in one block
- We can also assume that the first and last block read are cached!
  This can be used for a better calculation of IO seeks when we
  estimate the cost of a range index scan.

The parameter is fully implemented for MyISAM, Aria and InnoDB.
A separate patch will update handler::multi_range_read_info_const() to
take the benefits of this change and also remove the double
records_in_range() calls that are not anymore needed.

storage/heap/ha_heap.cc

@@ -579,8 +579,8 @@ int ha_heap::rename_table(const char * from, const char * to)
 }
 
 
-ha_rows ha_heap::records_in_range(uint inx, key_range *min_key,
-                                  key_range *max_key)
+ha_rows ha_heap::records_in_range(uint inx, const key_range *min_key,
+                                  const key_range *max_key, page_range *pages)
 {
   KEY *key=table->key_info+inx;
   if (key->algorithm == HA_KEY_ALG_BTREE)

storage/heap/ha_heap.h

@@ -102,7 +102,8 @@ public:
   int disable_indexes(uint mode);
   int enable_indexes(uint mode);
   int indexes_are_disabled(void);
-  ha_rows records_in_range(uint inx, key_range *min_key, key_range *max_key);
+  ha_rows records_in_range(uint inx, const key_range *start_key,
+                           const key_range *end_key, page_range *pages);
   int delete_table(const char *from);
   void drop_table(const char *name);
   int rename_table(const char * from, const char * to);

storage/heap/hp_hash.c

@@ -55,8 +55,9 @@ static ulong hp_hashnr(HP_KEYDEF *keydef, const uchar *key);
 			the range.
 */
 
-ha_rows hp_rb_records_in_range(HP_INFO *info, int inx,  key_range *min_key,
-                               key_range *max_key)
+ha_rows hp_rb_records_in_range(HP_INFO *info, int inx,
+                               const key_range *min_key,
+                               const key_range *max_key)
 {
   ha_rows start_pos, end_pos;
   HP_KEYDEF *keyinfo= info->s->keydef + inx;