DS-MRR support improvements (MWL#123, MWL#124, MWL#125)

- Lots of TODO comments
- add mrr_sort_keys flag to @@optimizer_switch
- [from Igor] SQL layer part passes HA_MRR_MATERIALIZED_KEYS flag
- Don't call rnd_pos() many times in a row if sorted rowid buffer
  has the same rowid value for multiple consequive (rowid, range_id) pairs.

sql/sql_join_cache.cc

@@ -651,6 +651,9 @@ int JOIN_CACHE_BKA::init()
 
   use_emb_key= check_emb_key_usage();
 
+  if (use_emb_key)
+    mrr_mode|= HA_MRR_MATERIALIZED_KEYS;
+
   create_remaining_fields(FALSE);
 
   set_constants();
@@ -2631,6 +2634,8 @@ int JOIN_CACHE_BKA_UNIQUE::init()
       data_fields_offset+= copy->length;
   } 
 
+  mrr_mode|= HA_MRR_MATERIALIZED_KEYS;
+
   DBUG_RETURN(rc);
 }