BUG#20850: Assert during slave shutdown in many rpl_* tests.
  This was caused by a race condition at the end of handle_slave_io
  which under some circumstances allowed the cleanup to proceed before
  the thread had completed.


sql/slave.cc:
  BUG#20850: Assert during slave shutdown in many rpl_* tests.
  This was caused by a race condition at the end of handle_slave_io
  which under some circumstances allowed the cleanup to proceed before
  the thread had completed.