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_utils.cc

@@ -25,6 +25,7 @@
 #include "wsrep_api.h"
 #include "wsrep_utils.h"
 #include "wsrep_mysqld.h"
+#include "wsrep_thd.h"
 
 #include <sql_class.h>
 
@@ -421,7 +422,8 @@ thd::thd (my_bool won) : init(), ptr(new THD(0))
   if (ptr)
   {
     ptr->thread_stack= (char*) &ptr;
-    ptr->store_globals();
+    wsrep_assign_from_threadvars(ptr);
+    wsrep_store_threadvars(ptr);
     ptr->variables.option_bits&= ~OPTION_BIN_LOG; // disable binlog
     ptr->variables.wsrep_on= won;
     ptr->security_ctx->master_access= ~(ulong)0;