Original idea from Zardosht Kasheff to add HA_CLUSTERED_INDEX

- Added a lot of code comments
- Updated get_best_ror_intersec() to prefer index scan on not clustered keys before clustered keys.
- Use HA_CLUSTERED_INDEX to define if one should use HA_MRR_INDEX_ONLY
- For test of using index or filesort to resolve ORDER BY, use HA_CLUSTERED_INDEX flag instead of primary_key_is_clustered()
- Use HA_TABLE_SCAN_ON_INDEX instead of primary_key_is_clustered() to decide if ALTER TABLE ... ORDER BY will have any effect.

sql/ha_partition.h:
  Added comment with warning for code unsafe to use with multiple storage engines at the same time
sql/handler.h:
  Added HA_CLUSTERED_INDEX.
  Documented primary_key_is_clustered()
sql/opt_range.cc:
  Added code comments
  Updated get_best_ror_intersec() to ignore clustered keys.
  Optimized away cpk_scan_used and one instance of current_thd (Simpler code)
  Use HA_CLUSTERED_INDEX to define if one should use HA_MRR_INDEX_ONLY
sql/sql_select.cc:
  Changed comment to #ifdef
  For test of using index or filesort to resolve ORDER BY, use HA_CLUSTERED_INDEX flag instead of primary_key_is_clustered()
  (Change is smaller than what it looks beause of indentation change)
sql/sql_table.cc:
  Use HA_TABLE_SCAN_ON_INDEX instead of primary_key_is_clustered() to decide if ALTER TABLE ... ORDER BY will have any effect.
storage/innobase/handler/ha_innodb.h:
  Added support for HA_CLUSTERED_INDEX
storage/innodb_plugin/handler/ha_innodb.cc:
  Added support for HA_CLUSTERED_INDEX
storage/xtradb/handler/ha_innodb.cc:
  Added support for HA_CLUSTERED_INDEX

sql/handler.h

@@ -161,8 +161,11 @@
 */
 #define HA_KEY_SCAN_NOT_ROR     128 
 #define HA_DO_INDEX_COND_PUSHDOWN  256 /* Supports Index Condition Pushdown */
-
-
+/*
+  Data is clustered on this key. This means that when you read the key
+  you also get the row data in the same block.
+*/
+#define HA_CLUSTERED_INDEX      512
 
 /*
   bits in alter_table_flags:
@@ -2311,9 +2314,22 @@ public:
 
 
  /*
-   @retval TRUE   Primary key (if there is one) is clustered
-                  key covering all fields
-   @retval FALSE  otherwise
+   Check if the primary key (if there is one) is a clustered key covering
+   all fields. This means:
+
+   - Data is stored together with the primary key (no secondary lookup
+     needed to find the row data). The optimizer uses this to find out
+     the cost of fetching data.
+   - The primary key is part of each secondary key and is used
+     to find the row data in the primary index when reading trough
+     secondary indexes.
+   - When doing a HA_KEYREAD_ONLY we get also all the primary key parts
+     into the row. This is critical property used by index_merge.
+
+   For a clustered primary key, index_flags() returns also HA_CLUSTERED_INDEX
+
+   @retval TRUE   yes
+   @retval FALSE  No.
  */
  virtual bool primary_key_is_clustered() { return FALSE; }
  virtual int cmp_ref(const uchar *ref1, const uchar *ref2)