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/join.test

@@ -692,13 +692,13 @@ create table t2 (a int, b int, primary key(a));
 insert into t2 select @v:=A.a+10*B.a, @v  from t1 A, t1 B;
 
 explain select * from t1;
-show status like '%cost%';
+show status like 'Last_query_cost';
 select 'The cost of accessing t1 (dont care if it changes' '^';
 
 select 'vv: Following query must use ALL(t1), eq_ref(A), eq_ref(B): vv' Z;
 
 explain select * from t1, t2 A, t2 B where A.a = t1.a and B.a=A.b;
-show status like '%cost%';
+show status like 'Last_query_cost';
 select '^^: The above should be ~= 20 + cost(select * from t1). Value less than 20 is an error' Z;