add 'afr' for user to set wrok acceptable failure ratio in configure file

add 'afr' for user to set different acceptable failure ratio for <work> in a configure file.
The unit is samples per one million, default is 0.
eg.  <work name="work12" workers="16" runtime="300" afr="500"> 
it means the acceptable failure ratio for this <work> is 0.0005 (= 500 / 1000000)