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_nested.result

@@ -1744,9 +1744,9 @@ ON t4.carrier = t1.carrier;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	index	package_id	package_id	5	NULL	45	Using where; Using index
 1	SIMPLE	t1	eq_ref	PRIMARY	PRIMARY	4	test.t2.package_id	1	
-1	SIMPLE	t3	ref	package_id	package_id	5	test.t2.package_id	1	Using index
 1	SIMPLE	t4	eq_ref	PRIMARY,id	PRIMARY	2	test.t1.carrier	1	Using where
 1	SIMPLE	t5	ref	carrier_id	carrier_id	5	test.t4.id	22	Using index
+1	SIMPLE	t3	ref	package_id	package_id	5	test.t2.package_id	1	Using index
 SELECT COUNT(*) 
 FROM ((t2 JOIN t1 ON t2.package_id = t1.id) 
 JOIN t3 ON t3.package_id = t1.id)