MDEV-19746: Galera test failures because of wsrep_slave_threads identification

Problem was that tests select INFORMATION_SCHEMA.PROCESSLIST processes
from user system user and empty state. Thus, there is not clear
state for slave threads.

Changes:
- Added new status variables that store current amount of applier threads
(wsrep_applier_thread_count) and rollbacker threads
(wsrep_rollbacker_thread_count). This will make clear how many slave threads
of certain type there is.
- Added THD state "wsrep applier idle" when applier slave thread is
waiting for work. This makes finding slave/applier threads easier.
- Added force-restart option for mtr to always restart servers between tests
to avoid race on start of the test
- Added wait_condition_with_debug to wait until the passed statement returns
true, or the operation times out. If operation times out, the additional error
statement will be executed

Changes to be committed:
	new file:   mysql-test/include/force_restart.inc
	new file:   mysql-test/include/wait_condition_with_debug.inc
	modified:   mysql-test/mysql-test-run.pl
	modified:   mysql-test/suite/galera/disabled.def
	modified:   mysql-test/suite/galera/r/MW-336.result
	modified:   mysql-test/suite/galera/r/galera_kill_applier.result
	modified:   mysql-test/suite/galera/r/galera_var_slave_threads.result
	new file:   mysql-test/suite/galera/t/MW-336.cnf
	modified:   mysql-test/suite/galera/t/MW-336.test
	modified:   mysql-test/suite/galera/t/galera_kill_applier.test
	modified:   mysql-test/suite/galera/t/galera_parallel_autoinc_largetrx.test
	modified:   mysql-test/suite/galera/t/galera_parallel_autoinc_manytrx.test
	modified:   mysql-test/suite/galera/t/galera_var_slave_threads.test
	modified:   mysql-test/suite/wsrep/disabled.def
	modified:   mysql-test/suite/wsrep/r/variables.result
	modified:   mysql-test/suite/wsrep/t/variables.test
	modified:   sql/mysqld.cc
	modified:   sql/wsrep_mysqld.cc
	modified:   sql/wsrep_mysqld.h
	modified:   sql/wsrep_thd.cc
	modified:   sql/wsrep_var.cc

mysql-test/include/wait_condition_with_debug.inc

@@ -0,0 +1,69 @@
+# include/wait_condition_with_debug.inc
+#
+# SUMMARY
+#
+#    Waits until the passed statement returns true, or the operation
+#    times out.  If the operation times out, the additional error
+#    statement will be executed.
+#
+# USAGE
+#
+#    let $wait_condition=
+#      SELECT c = 3 FROM t;
+#    let $wait_condition_on_error_output= select count(*) from t;
+#    [let $explicit_default_wait_timeout= N] # to override the default reset
+#    --source include/wait_condition_with_debug.inc
+#
+#   OR
+#
+#    let $wait_timeout= 60; # Override default 30 seconds with 60.
+#    let $wait_condition=
+#      SELECT c = 3 FROM t;
+#    let $wait_condition_on_error_output= select count(*) from t;
+#    --source include/wait_condition_with_debug.inc
+#    --echo Executed the test condition $wait_condition_reps times
+#
+#
+# EXAMPLE
+#    events_bugs.test, events_time_zone.test
+#
+
+let $wait_counter= 300;
+if ($wait_timeout)
+{
+  let $wait_counter= `SELECT $wait_timeout * 10`;
+}
+# Reset $wait_timeout so that its value won't be used on subsequent
+# calls, and default will be used instead.
+if ($explicit_default_wait_timeout)
+{
+  --let $wait_timeout= $explicit_default_wait_timeout
+}
+if (!$explicit_default_wait_timeout)
+{
+  --let $wait_timeout= 0
+}
+
+# Keep track of how many times the wait condition is tested
+# This is used by some tests (e.g., main.status)
+let $wait_condition_reps= 0;
+while ($wait_counter)
+{
+    --error 0,ER_NO_SUCH_TABLE,ER_LOCK_WAIT_TIMEOUT,ER_UNKNOWN_COM_ERROR,ER_LOCK_DEADLOCK
+    let $success= `$wait_condition`;
+    inc $wait_condition_reps;
+    if ($success)
+    {
+        let $wait_counter= 0;
+    }
+    if (!$success)
+    {
+        real_sleep 0.1;
+        dec $wait_counter;
+    }
+}
+if (!$success)
+{
+  echo Timeout in wait_condition.inc for $wait_condition;
+  --eval $wait_condition_on_error_output
+}