If rpl_semi_sync_master_timeout is large, calculation of absolute waiting time in semi-sync plugin is inefficient. This error is specific to systems with 64 bit long values (all 64 bit Unixes)
In  rpl_semi_sync_master_timeout has maximal value (= MAX_ULONGLONG), calculating abstime may require  ~ 18 billion subtract operations.

The fix is to use division instead of subtraction-in-a-loop. Also fixed an integer overflow bug.