I/O thread Code optimization, reduces the number of OP_WRITE or OP_READ judgments

[dupengchuan]

if there are n clients per thread, the judgment is O(n),this optimization will reduce it to O(1)

[CLAassistant]

All committers have signed the CLA.

[sundb]

thanks for the minor improvement, but I'm not sure if modern compilers do the optimization for us.
please wait for me to take some time to check it.

[sundb]

this does save judment from the assembly.
although dont see benefit from benchmark, still agree with this change.

[dupengchuan]

Kindly ping @oranagra @yossigo

[oranagra]

what are we saving here? moving the if outside the loop, i seriously doubt you'll be able to measure the impact of that (you're welcome to prove otherwise).
if it would have also resulted in drastically cleaner code, i'd consider anyway, but i think it goes the other way (more code).