to leave

The artifact was caused by
a flaw in concurrent accessing the slave's io thd by
the io itself and a handling show slave status thread.
Namely, show_master_info did not acquire mi->run_lock mutex that is
specified for mi->io_thd member.

Fixed with deploying the mutex locking and unlocking. The mutex is kept
short time and without interleaving with mi->data_lock mutex.

Todo: to report and fix an issue with 
    sys_var_slave_skip_counter::{methods} 
seem to acquire incorrectly
     active_mi->rli.run_lock
instead of the specified
     active_mi->rli.data_lock

A test case is difficult to compose, so rpl_packet should continue serving
as the indicator.


sql/slave.cc:
  implementing a TODO left at 4.1 time:
  mending access to mi->io_thd with the specified mutex;
sql/slave.h:
  adding a member name to the list of that run_lock guards.