Set dictGetFairRandomKey() samples to 20 for final version.
Set dictGetFairRandomKey() samples to 20 for final version.

Distribution improves dramatically: tests show it clearly. Better to
have a slower implementation than a wrong one, because random member
extraction should be correct or tends to be useless for a number of
tasks.

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis