LRU eviction pool implementation.

This is an improvement over the previous eviction algorithm where we use
an eviction pool that is persistent across evictions of keys, and gets
populated with the best candidates for evictions found so far.

It allows to approximate LRU eviction at a given number of samples
better than the previous algorithm used.

src/redis.h

@@ -411,13 +411,30 @@ typedef struct redisObject {
     _var.ptr = _ptr; \
 } while(0);
 
+/* To improve the quality of the LRU approximation we take a set of keys
+ * that are good candidate for eviction across freeMemoryIfNeeded() calls.
+ *
+ * Entries inside the eviciton pool are taken ordered by idle time, putting
+ * greater idle times to the right (ascending order).
+ *
+ * Empty entries have the key pointer set to NULL. */
+#define REDIS_EVICTION_POOL_SIZE 16
+struct evictionPoolEntry {
+    unsigned long long idle;    /* Object idle time. */
+    sds key;                    /* Key name. */
+};
+
+/* Redis database representation. There are multiple databases identified
+ * by integers from 0 (the default database) up to the max configured
+ * database. The database number is the 'id' field in the structure. */
 typedef struct redisDb {
     dict *dict;                 /* The keyspace for this DB */
     dict *expires;              /* Timeout of keys with a timeout set */
     dict *blocking_keys;        /* Keys with clients waiting for data (BLPOP) */
     dict *ready_keys;           /* Blocked keys that received a PUSH */
     dict *watched_keys;         /* WATCHED keys for MULTI/EXEC CAS */
-    int id;
+    struct evictionPoolEntry *eviction_pool;    /* Eviction pool of keys */
+    int id;                     /* Database ID */
     long long avg_ttl;          /* Average TTL, just for stats */
 } redisDb;