MDEV-3999: Valgrind errors 'invalid write' or assorted server crashes on concurrent flow with partitioned Aria tables
MDEV-3989: Server crashes on import from MariaDB mysqldump export with partitioned Aria table.

Problem was that bulk insert in aria was not properly protected against concurrent selects.


storage/maria/ha_maria.cc:
  Move settings of file->state to _ma_block_start_trans() to ensure that lock_key_trees is not changed by a concurrent bulk_insert.
storage/maria/ma_check.c:
  Added DBUG_ASSERT()
storage/maria/ma_open.c:
  Set start_trans to ma_start_trans for default behaviour.
storage/maria/ma_pagecrc.c:
  Removed test for 'non_transactional' as a now_transactinal could be reset while a flush was happening.
storage/maria/ma_state.c:
  Moved setting of info->state from external_lock to start_trans to protect against concurrent running bulk inserts.
  This works as the other threads will wait in thr_lock() until bulk_insert is done and keys are re-generated.
storage/maria/ma_state.h:
  Added _ma_start_trans()