MDEV-8208: Sporadic SEGFAULT on startup

Problem:
When mysqld starts as a galera node, it creates 2 system threads
(applier & rollbacker) using start_wsrep_THD(). These threads are
created before plugin initialization (plugin_init()) for SST methods
like rsync and xtrabackup.

The threads' initialization itself can proceed in parallel to mysqld's
main thread of execution. As a result, the thread initialization code
(start_wsrep_THD()) can end up accessing some un/partially initialized
structures (like maria_hton, in this particular case) resulting in
segfault.

Solution:
Fixed by calling THD::init_for_queries() (which accesses maria_hton)
only after the plugins have been initialized.

sql/wsrep_mysqld.cc

@@ -65,6 +65,13 @@ my_bool wsrep_restart_slave_activated  = 0; // node has dropped, and slave
                                             // restart will be needed
 my_bool wsrep_slave_UK_checks          = 0; // slave thread does UK checks
 my_bool wsrep_slave_FK_checks          = 0; // slave thread does FK checks
+
+/*
+  Set during the creation of first wsrep applier and rollback threads.
+  Since these threads are critical, abort if the thread creation fails.
+*/
+my_bool wsrep_creating_startup_threads = 0;
+
 /*
  * End configuration options
  */
@@ -700,6 +707,7 @@ void wsrep_init_startup (bool first)
 
   if (!wsrep_start_replication()) unireg_abort(1);
 
+  wsrep_creating_startup_threads= 1;
   wsrep_create_rollbacker();
   wsrep_create_appliers(1);