MW-357 Reset thd->wsrep_apply_toi regardless of applier exiting

Applier does not reset thd->wsrep_apply_toi if applier thread decides
to exit by setting 'exit= true'. The problem is that galera side may
decide not to kill the applier thread: for instance if we try to
SET wsrep_slave_threads = 0; then galera refuses to kill the last
applier thread. If this happens we are left with a thd which has not
been reset to the initial state.
This patch ensures that the thd is reset regardless of the applier
thread exiting or not.

mysql-test/suite/galera/r/MW-357.result

+SET GLOBAL wsrep_slave_threads = 0;
+Warnings:
+Warning	1292	Truncated incorrect wsrep_slave_threads value: '0'
+CREATE TABLE t1 (f1 INT PRIMARY KEY) Engine=InnoDB;
+INSERT INTO t1 VALUES (1);
+DROP TABLE t1;

mysql-test/suite/galera/t/MW-357.test

+--source include/galera_cluster.inc
+--source include/have_innodb.inc
+
+--connection node_2
+SET GLOBAL wsrep_slave_threads = 0;
+
+--connection node_1
+CREATE TABLE t1 (f1 INT PRIMARY KEY) Engine=InnoDB;
+
+--connection node_1
+INSERT INTO t1 VALUES (1);
+
+DROP TABLE t1;

sql/wsrep_applier.cc

@@ -217,6 +217,8 @@ wsrep_cb_status_t wsrep_apply_cb(void* const             ctx,
 {
   THD* const thd((THD*)ctx);
 
+  assert(thd->wsrep_apply_toi == false);
+
   // Allow tests to block the applier thread using the DBUG facilities.
   DBUG_EXECUTE_IF("sync.wsrep_apply_cb",
                  {
@@ -380,7 +382,7 @@ wsrep_cb_status_t wsrep_commit_cb(void*         const     ctx,
     mysql_mutex_unlock(&LOCK_wsrep_slave_threads);
   }
 
-  if (*exit == false && thd->wsrep_applier)
+  if (thd->wsrep_applier)
   {
     /* From trans_begin() */
     thd->variables.option_bits|= OPTION_BEGIN;