Draft #1 of a new expired keys collection algorithm.

The main idea here is that when we are no longer to expire keys at the
rate the are created, we can't block more in the normal expire cycle as
this would result in too big latency spikes.

For this reason the commit introduces a "fast" expire cycle that does
not run for more than 1 millisecond but is called in the beforeSleep()
hook of the event loop, so much more often, and with a frequency bound
to the frequency of executed commnads.

The fast expire cycle is only called when the standard expiration
algorithm runs out of time, that is, consumed more than
REDIS_EXPIRELOOKUPS_TIME_PERC of CPU in a given cycle without being able
to take the number of already expired keys that are yet not collected
to a number smaller than 25% of the number of keys.

You can test this commit with different loads, but a simple way is to
use the following:

Extreme load with pipelining:

redis-benchmark -r 100000000 -n 100000000  \
        -P 32 set ele:rand:000000000000 foo ex 2

Remove the -P32 in order to avoid the pipelining for a more real-world
load.

In another terminal tab you can monitor the Redis behavior with:

redis-cli -i 0.1 -r -1 info keyspace

and

redis-cli --latency-history

Note: this commit will make Redis printing a lot of debug messages, it
is not a good idea to use it in production.

src/redis.h

@@ -74,7 +74,7 @@
 #define REDIS_MAXIDLETIME       0       /* default client timeout: infinite */
 #define REDIS_DEFAULT_DBNUM     16
 #define REDIS_CONFIGLINE_MAX    1024
-#define REDIS_EXPIRELOOKUPS_PER_CRON    10 /* lookup 10 expires per loop */
+#define REDIS_EXPIRELOOKUPS_PER_CRON   20 /* lookup 20 expires per loop */
 #define REDIS_EXPIRELOOKUPS_TIME_PERC   25 /* CPU max % for keys collection */
 #define REDIS_DBCRON_DBS_PER_CALL 16
 #define REDIS_MAX_WRITE_PER_EVENT (1024*64)
@@ -744,7 +744,8 @@ struct redisServer {
     size_t set_max_intset_entries;
     size_t zset_max_ziplist_entries;
     size_t zset_max_ziplist_value;
-    time_t unixtime;        /* Unix time sampled every second. */
+    time_t unixtime;        /* Unix time sampled every cron cycle. */
+    long long mstime;       /* Like 'unixtime' but with milliseconds resolution. */
     /* Pubsub */
     dict *pubsub_channels;  /* Map channels to list of subscribed clients */
     list *pubsub_patterns;  /* A list of pubsub_patterns */