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_server_service.h

@@ -77,5 +77,14 @@ private:
   Wsrep_server_state& m_server_state;
 };
 
+/**
+   Helper method to create new streaming applier.
+
+   @param orig_thd Original thd context to copy operation context from.
+   @param ctx Context string for debug logging.
+ */
+class Wsrep_applier_service;
+Wsrep_applier_service*
+wsrep_create_streaming_applier(THD *orig_thd, const char *ctx);
 
 #endif /* WSREP_SERVER_SERVICE */