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.

include/my_base.h

@@ -636,6 +636,17 @@ typedef struct st_key_multi_range
 } KEY_MULTI_RANGE;
 
 
+/* Store first and last leaf page accessed by records_in_range */
+
+typedef struct st_page_range
+{
+  ulonglong first_page;
+  ulonglong last_page;
+} page_range;
+
+#define UNUSED_PAGE_NO ULONGLONG_MAX
+#define unused_page_range { UNUSED_PAGE_NO, UNUSED_PAGE_NO }
+
 /* For number of records */
 #ifdef BIG_TABLES
 #define rows2double(A)	ulonglong2double(A)