A test case was waiting for a fixed number of seconds for a specific
state of the slave IO thread to take place.
Fixed by waiting in a loop for that specific thread state instead
(or timeout).
thd->enter_cond() and exit_cond(), so that the I/O thread accepts to stop
when it's waiting for relay log space.
Reset ignore_log_space_limit to 0 when the SQL thread terminates.
