MDEV-19826 10.4 seems to crash with "pool-of-threads" (#1370)

MariaDB 10.4 was crashing when thread-handling was set to
pool-of-threads and wsrep was enabled.

There were two apparent reasons for the crash:
- Connection handling in threadpool_common.cc was missing calls to
  control wsrep client state.
- Thread specific storage which contains thread variables (THR_KEY_mysys)
  was not handled appropriately by wsrep patch when pool-of-threads
  was configured.

This patch addresses the above issues in the following way:
- Wsrep client state open/close was moved in thd_prepare_connection() and
  end_connection() to have common handling for one-thread-per-connection
  and pool-of-threads.
- Thread local storage handling in wsrep patch was reworked by introducing
  set of wsrep_xxx_threadvars() calls which replace calls to
  THD store_globals()/reset_globals() and deal with thread handling
  specifics internally.

Wsrep-lib was updated to version which relaxes internal concurrency
related sanity checks.

Rollback code from wsrep_rollback_process() was extracted to separate calls
for better readability.

Post rollback thread was removed as it was completely unused.

sql/threadpool_common.cc

@@ -23,7 +23,9 @@
 #include <sql_audit.h>
 #include <debug_sync.h>
 #include <threadpool.h>
-
+#ifdef WITH_WSREP
+#include "wsrep_trans_observer.h"
+#endif /* WITH_WSREP */
 
 /* Threadpool parameters */
 
@@ -137,6 +139,11 @@ static inline void set_thd_idle(THD *thd)
 */
 static void thread_attach(THD* thd)
 {
+#ifdef WITH_WSREP
+  /* Wait until possible background rollback has finished before
+     attaching the thd. */
+  wsrep_wait_rollback_complete_and_acquire_ownership(thd);
+#endif /* WITH_WSREP */
   pthread_setspecific(THR_KEY_mysys,thd->mysys_var);
   thd->thread_stack=(char*)&thd;
   thd->store_globals();