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.


mysql-test/r/rpl_relayspace.result:
  result update
mysql-test/t/rpl_relayspace-slave.opt:
  smaller relay_log_space_limit to speed up things.
mysql-test/t/rpl_relayspace.test:
  Less queries in the test, to make it simpler.
  Testcase if the IO thread blocked in wait_for_relay_log_space
  accepts to stop when STOP SLAVE.
sql/slave.cc:
  thd->enter_cond() and exit_cond(), so that the I/O thread accepts to stop
  when it's waiting on cond (waiting for relay log space).
  Reset ignore_log_space_limit to 0 when the SQL thread terminates.