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/example/ha_example.cc

@@ -839,6 +839,10 @@ int ha_example::delete_table(const char *name)
   @brief
   Given a starting key and an ending key, estimate the number of rows that
   will exist between the two keys.
+  The handler can also optionally update the 'pages' parameter with the page
+  number that contains the min and max keys. This will help the optimizer
+  to know if two ranges are partly on the same pages and if the min and
+  max key are on the same page.
 
   @details
   end_key may be empty, in which case determine if start_key matches any rows.
@@ -848,8 +852,10 @@ int ha_example::delete_table(const char *name)
   @see
   check_quick_keys() in opt_range.cc
 */
-ha_rows ha_example::records_in_range(uint inx, key_range *min_key,
-                                     key_range *max_key)
+ha_rows ha_example::records_in_range(uint inx,
+                                     const key_range *min_key,
+                                     const key_range *max_key,
+                                     page_range *pages)
 {
   DBUG_ENTER("ha_example::records_in_range");
   DBUG_RETURN(10);                         // low number to force index usage