Added EQ_REF chaining to the greedy_optimizer

MDEV-28073 Slow query performance in MariaDB when using many table

The idea is to prefer and chain EQ_REF tables (tables that uses an
unique key to find a row) when searching for the best table combination.
This significantly reduces row combinations that has to be examined.
This is optimization is enabled when setting optimizer_prune_level=2
(which is now default).

Implementation:
- optimizer_prune_level has a new level, 2, which enables EQ_REF
  optimization in addition to the pruning done by level 1.
  Level 2 is now default.
- Added JOIN::eq_ref_tables that contains bits of tables that could use
  potentially use EQ_REF access in the query.  This is calculated
  in sort_and_filter_keyuse()

Under optimizer_prune_level=2:
- When the greedy_optimizer notices that the preceding table was an
  EQ_REF table, it tries to add an EQ_REF table next. If an EQ_REF
  table exists, only this one will be considered at this level.
  We also collect all EQ_REF tables chained by the next levels and these
  are ignored on the starting level as we have already examined these.
  If no EQ_REF table exists, we continue as normal.

This optimization speeds up the greedy_optimizer combination test with
~25%

Other things:
- I ported the changes in MySQL 5.7 to greedy_optimizer.test to MariaDB
  to be able to ensure we can handle all cases that MySQL can do.
- I have run all tests with --mysqld=--optimizer_prune_level=1 to verify that
  there where no test changes.

mysql-test/main/opt_trace_index_merge.result

@@ -203,23 +203,27 @@ explain select * from t1 where a=1 or b=1	{
             "considered_execution_plans": [
               {
                 "plan_prefix": [],
-                "table": "t1",
-                "best_access_path": {
-                  "considered_access_paths": [
-                    {
-                      "access_type": "index_merge",
-                      "resulting_rows": 2,
-                      "cost": 2.484903732,
-                      "chosen": true
+                "get_costs_for_tables": [
+                  {
+                    "best_access_path": {
+                      "table": "t1",
+                      "considered_access_paths": [
+                        {
+                          "access_type": "index_merge",
+                          "resulting_rows": 2,
+                          "cost": 2.484903732,
+                          "chosen": true
+                        }
+                      ],
+                      "chosen_access_method": {
+                        "type": "index_merge",
+                        "records": 2,
+                        "cost": 2.484903732,
+                        "uses_join_buffering": false
+                      }
                     }
-                  ],
-                  "chosen_access_method": {
-                    "type": "index_merge",
-                    "records": 2,
-                    "cost": 2.484903732,
-                    "uses_join_buffering": false
                   }
-                }
+                ]
               },
               {
                 "plan_prefix": [],