Addressed all remaining issues from the review of the patch

that introduced engine independent persistent statistics.
In particular:
- added an enumeration type for possible values of the system
  variable use_stat_tables
- renamed KEY::real_rec_per_key to KEY::actual_rec_per_key
- optimized the collection of statistical data for any primary
  key defined only on one column.

sql/sql_delete.cc

@@ -35,6 +35,7 @@
 #include "sql_select.h"
 #include "sp_head.h"
 #include "sql_trigger.h"
+#include "sql_statistics.h"
 #include "transaction.h"
 #include "records.h"                            // init_read_record,
 #include "sql_derived.h"                        // mysql_handle_list_of_derived