Change cost for REF to take into account cost for 1 extra key read_next

The main difference in code path between EQ_REF and REF is that for
REF we have to do an extra read_next on the index to check that there
is no more matching rows.

Before this patch we added a preference of EQ_REF by ensuring that REF
would always estimate to find at least 2 rows.

This patch adds the cost of the extra key read_next to REF access and
removes the code that limited REF to at least 2 rows. For some queries
this can have a big effect as the total estimated rows will be halved
for each REF table with 1 rows.

multi_range cost calculations are also changed to take into account
the difference between EQ_REF and REF.

The effect of the patch to the test suite:
- About 80 test case changed
- Almost all changes where for EXPLAIN where estimated rows for REF
  where changed from 2 to 1.
- A few test cases using explain extended had a change of 'filtered'.
  This is because of the estimated rows are now closer to the
  calculated selectivity.
- A very few test had a change of table order.
  This is because the change of estimated rows from 2 to 1 or the small
  cost change for REF
  (main.subselect_sj_jcl6, main.group_by, main.dervied_cond_pushdown,
  main.distinct, main.join_nested, main.order_by, main.join_cache)
- No key statistics and the estimated rows are now smaller which cased
  estimated filtering to be lower.
  (main.subselect_sj_mat)
- The number of total rows are halved.
  (main.derived_cond_pushdown)
- Plans with 1 row changed to use RANGE instead of REF.
  (main.group_min_max)
- ALL changed to REF
  (main.key_diff)
- Key changed from ref + index_only to PRIMARY key for InnoDB, as
  OPTIMIZER_ROW_LOOKUP_COST + OPTIMIZER_ROW_NEXT_FIND_COST is smaller than
  OPTIMIZER_KEY_LOOKUP_COST + OPTIMIZER_KEY_NEXT_FIND_COST.
  (main.join_outer_innodb)
- Cost changes printouts
  (main.opt_trace*)
- Result order change
  (innodb_gis.rtree)

mysql-test/main/opt_trace_index_merge_innodb.result

@@ -118,7 +118,7 @@ explain select * from t1 where pk1 != 0  and key1 = 1	{
                         "using_mrr": false,
                         "index_only": false,
                         "rows": 1000,
-                        "cost": 0.19579056,
+                        "cost": 0.19598856,
                         "chosen": false,
                         "cause": "cost"
                       },
@@ -129,7 +129,7 @@ explain select * from t1 where pk1 != 0  and key1 = 1	{
                         "using_mrr": false,
                         "index_only": false,
                         "rows": 1,
-                        "cost": 0.00415068,
+                        "cost": 0.00424968,
                         "chosen": true
                       }
                     ],
@@ -168,7 +168,7 @@ explain select * from t1 where pk1 != 0  and key1 = 1	{
                       "ranges": ["(1) <= (key1) <= (1)"]
                     },
                     "rows_for_plan": 1,
-                    "cost_for_plan": 0.00415068,
+                    "cost_for_plan": 0.00424968,
                     "chosen": true
                   }
                 }
@@ -216,7 +216,7 @@ explain select * from t1 where pk1 != 0  and key1 = 1	{
                           "index": "key1",
                           "used_range_estimates": true,
                           "rows": 1,
-                          "cost": 0.00335956,
+                          "cost": 0.00345856,
                           "chosen": true
                         },
                         {
@@ -229,7 +229,7 @@ explain select * from t1 where pk1 != 0  and key1 = 1	{
                         "type": "ref",
                         "rows_read": 1,
                         "rows_out": 1,
-                        "cost": 0.00335956,
+                        "cost": 0.00345856,
                         "uses_join_buffering": false
                       }
                     }
@@ -240,14 +240,14 @@ explain select * from t1 where pk1 != 0  and key1 = 1	{
                 "plan_prefix": "",
                 "table": "t1",
                 "rows_for_plan": 1,
-                "cost_for_plan": 0.00335956
+                "cost_for_plan": 0.00345856
               }
             ]
           },
           {
             "best_join_order": ["t1"],
             "rows": 1,
-            "cost": 0.00335956
+            "cost": 0.00345856
           },
           {
             "substitute_best_equal": {