activeExpireCycle(): process only a small number of DBs per iteration.
activeExpireCycle(): process only a small number of DBs per iteration.

This small number of DBs is set to 16 so actually in the default
configuraiton Redis should behave exactly like in the past.
However the difference is that when the user configures a very large
number of DBs we don't do an O(N) operation, consuming a non trivial
amount of CPU per serverCron() iteration.

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