Fixed slow bootstrap introduced in 10.6

The problem was that the signal thread was not killed when using
unireg_abort().

The bug was introduced by:
MDEV-30260: Slave crashed:reload_acl_and_cache during shutdown

Other things fixed:
- Don't produce memory leaks with safemalloc if all threads was not
  ended properly (not useful)

sql/mysqld.cc

@@ -1889,6 +1889,7 @@ extern "C" void unireg_abort(int exit_code)
   wsrep_sst_auth_free();
 #endif // WITH_WSREP
 
+  wait_for_signal_thread_to_end();
   clean_up(!opt_abort && (exit_code || !opt_bootstrap)); /* purecov: inspected */
   DBUG_PRINT("quit",("done with cleanup in unireg_abort"));
   mysqld_exit(exit_code);