MDEV-18648: slave_parallel_mode= optimistic default in 10.5

Description:
============
To change 'CONSERVATIVE' @@global.slave_parallel_mode default to 'OPTIMISTIC'
in 10.5.

@sql/sys_vars.cc
Changed default parallel_mode to 'OPTIMISTIC'

@sql/rpl_filter.cc
Changed default parallel_mode to 'OPTIMISTIC'

@sql/mysqld.cc
Removed the initialization of 'SLAVE_PARALLEL_CONSERVATIVE' to
'opt_slave_parallel_mode' variable.

@mysql-test/suite/rpl/t/rpl_parallel_mdev6589.test
@mysql-test/suite/rpl/t/rpl_mdev6386.test
Added 'mtr' suppression to ignore 'ER_PRIOR_COMMIT_FAILED'. In case of
'OPTIMISTIC' mode if a transaction gets killed during "wait_for_prior_commit"
it results in above error "1964". Hence suppression needs to be added for this
error.

@mysql-test/suite/rpl/t/rpl_parallel_conflicts.test
Test has a 'slave.opt' which explicitly sets slave_parallel_mode to
'conservative'. When the test ends this mode conflicts with new default mode.
Hence check test case reports an error. The 'slave.opt' is removed and options
are set and reset within test.

@mysql-test/suite/multi_source/info_logs.result
@mysql-test/suite/multi_source/reset_slave.result
@mysql-test/suite/multi_source/simple.result
Result content mismatch in "show slave status" output. This is expected as new
slave_parallel_mode='OPTIMISTIC'.

@mysql-test/include/check-testcase.test
Updated default 'slave_parallel_mode' to 'optimistic'.

Refactored rpl_parallel.test into following test cases.
Test case 1: @mysql-test/suite/rpl/t/rpl_parallel_domain.test
Test case 2: @mysql-test/suite/rpl/t/rpl_parallel_domain_slave_single_grp.test
Test case 3: @mysql-test/suite/rpl/t/rpl_parallel_single_grpcmt.test
Test case 4: @mysql-test/suite/rpl/t/rpl_parallel_stop_slave.test
Test case 5: @mysql-test/suite/rpl/t/rpl_parallel_slave_bgc_kill.test
Test case 6: @mysql-test/suite/rpl/t/rpl_parallel_gco_wait_kill.test
Test case 7: @mysql-test/suite/rpl/t/rpl_parallel_free_deferred_event.test
Test case 8: @mysql-test/suite/rpl/t/rpl_parallel_missed_error_handling.test
Test case 9: @mysql-test/suite/rpl/t/rpl_parallel_innodb_lock_conflict.test
Test case 10: @mysql-test/suite/rpl/t/rpl_parallel_gtid_slave_pos_update_fail.test
Test case 11: @mysql-test/suite/rpl/t/rpl_parallel_wrong_exec_master_pos.test
Test case 12: @mysql-test/suite/rpl/t/rpl_parallel_partial_binlog_trans.test
Test case 13: @mysql-test/suite/rpl/t/rpl_parallel_ignore_error_on_rotate.test
Test case 14: @mysql-test/suite/rpl/t/rpl_parallel_wrong_binlog_order.test
Test case 15: @mysql-test/suite/rpl/t/rpl_parallel_incorrect_relay_pos.test
Test case 16: @mysql-test/suite/rpl/t/rpl_parallel_retry_deadlock.test
Test case 17: @mysql-test/suite/rpl/t/rpl_parallel_deadlock_corrupt_binlog.test
Test case 18: @mysql-test/suite/rpl/t/rpl_parallel_mode.test
Test case 19: @mysql-test/suite/rpl/t/rpl_parallel_analyze_table_hang.test
Test case 20: @mysql-test/suite/rpl/t/rpl_parallel_record_gtid_wakeup.test
Test case 21: @mysql-test/suite/rpl/t/rpl_parallel_stop_on_con_kill.test
Test case 22: @mysql-test/suite/rpl/t/rpl_parallel_rollback_assert.test

mysql-test/suite/rpl/t/rpl_parallel_conflicts.test

@@ -15,7 +15,12 @@ CREATE TABLE t8 (a int PRIMARY KEY, b INT) ENGINE=InnoDB;
 --connection server_2
 --sync_with_master
 --source include/stop_slave.inc
-
+SET @old_mode= @@GLOBAL.slave_parallel_mode;
+SET GLOBAL slave_parallel_mode='conservative';
+SET @old_threads= @@GLOBAL.slave_parallel_threads;
+SET GLOBAL slave_parallel_threads=40;
+SET @old_transaction_retries= @@GLOBAL.slave_transaction_retries;
+SET GLOBAL slave_transaction_retries=5;
 
 # Using dbug error injection, we artificially create event groups with a lot of
 # conflicting transactions in each event group. The bugs were originally seen
@@ -249,6 +254,9 @@ SELECT * FROM t7 ORDER BY a;
 
 --source include/stop_slave.inc
 SET GLOBAL debug_dbug=@old_dbug;
+SET GLOBAL slave_parallel_mode=@old_mode;
+SET GLOBAL slave_parallel_threads=@old_threads;
+SET GLOBAL slave_transaction_retries=@old_transaction_retries;
 
 # Clean up.
 --source include/start_slave.inc