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/wsrep_client_service.cc

@@ -30,9 +30,9 @@
 #include "slave.h"   /* opt_log_slave_updates */
 #include "transaction.h" /* trans_commit()... */
 #include "log.h"      /* stmt_has_updated_trans_table() */
-//#include "debug_sync.h"
 #include "mysql/service_debug_sync.h"
 #include "mysql/psi/mysql_thread.h" /* mysql_mutex_assert_owner() */
+
 namespace
 {
 
@@ -57,16 +57,12 @@ Wsrep_client_service::Wsrep_client_service(THD* thd,
 
 void Wsrep_client_service::store_globals()
 {
-  DBUG_ENTER("Wsrep_client_service::store_globals");
-  m_thd->store_globals();
-  DBUG_VOID_RETURN;
+  wsrep_store_threadvars(m_thd);
 }
 
 void Wsrep_client_service::reset_globals()
 {
-  DBUG_ENTER("Wsrep_client_service::reset_globals");
-  m_thd->reset_globals();
-  DBUG_VOID_RETURN;
+  wsrep_reset_threadvars(m_thd);
 }
 
 bool Wsrep_client_service::interrupted(