Problem
-------
The Relay_Log_Space variable shown in SHOW SLAVE STATUS is
sometimes much higher than the actual disk space used by relay
logs.

Analysis
--------
This is because we are not writing to
Relay_log_info::log_space_total in a synchronized manner. i.e, no
lock is being taken by the IO thread while updating the variable.

Fix
---
The Relay_log_info::log_space_total is now guarded by the
Relay_log_info::log_space_lock and this protects concurrent
update on Relay_log_info::log_space_total.