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.

mysql-test/suite/galera/r/galera_performance_schema.result

@@ -7,7 +7,6 @@ WHERE name LIKE 'thread/sql/wsrep%'
 ORDER BY name;
 name	thread/sql/wsrep_applier_thread
 name	thread/sql/wsrep_rollbacker_thread
-name	thread/sql/wsrep_rollbacker_thread
 use test;
 create table t1 (a int not null primary key) engine=innodb;
 insert into t1 values (1),(2);