Redis CLOCK_MONOTONIC replace gettimeofday when use expire time in key

why not use clock_gettime(CLOCK_MONOTONIC, &ts); in function （long long ustime(void)）

Comment From: WiFeng

There is discussion very long time ago and explained why. #416

Comment From: xiaozhitaba

ok thank you！！！

Comment From: xiaozhitaba

same as ？#7004