activeExpireCycle() smarter with many DBs and under expire pressure.
activeExpireCycle() tries to test just a few DBs per iteration so that it scales if there are many configured DBs in the Redis instance. However this commit makes it a bit smarter when one a few of those DBs are under expiration pressure and there are many many keys to expire. What we do is to remember if in the last iteration had to return because we ran out of time. In that case the next iteration we'll test all the configured DBs so that we are sure we'll test again the DB under pressure. Before of this commit after some mass-expire in a given DB the function tested just a few of the next DBs, possibly empty, a few per iteration, so it took a long time for the function to reach again the DB under pressure. This resulted in a lot of memory being used by already expired keys and never accessed by clients.
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