better fix for MySQL bugs

BUG#26447 prefer a clustered key for an index scan, as secondary index is always slower
  ... which was fixed to cause
BUG#35850 queries take 50% longer
  ... and was reverted

and

BUG#39653 prefer a secondary index for an index scan, as clustered key is always slower
  ... which was fixed to cause
BUG#55656 mysqldump takes six days instead of half an hour
  ... and was amended with a special case workaround


sql/opt_range.cc:
  move get_index_only_read_time() into the handler class
sql/sql_select.cc:
  use cost not an index length when choosing a cheaper index

sql/handler.h

@@ -1304,6 +1304,16 @@ public:
   { return ulonglong2double(stats.data_file_length) / IO_SIZE + 2; }
   virtual double read_time(uint index, uint ranges, ha_rows rows)
   { return rows2double(ranges+rows); }
+
+  /**
+    Calculate cost of 'keyread' scan for given index and number of records.
+
+     @param index    index to read
+     @param ranges   #of ranges to read
+     @param rows     #of records to read
+  */
+  virtual double keyread_time(uint index, uint ranges, ha_rows rows);
+
   virtual const key_map *keys_to_use_for_scanning() { return &key_map_empty; }
   bool has_transactions()
   { return (ha_table_flags() & HA_NO_TRANSACTIONS) == 0; }