RDB: Ability to save LFU/LRU info.

This is a big win for caching use cases, since on reloading Redis will
still have some idea about what is worth to evict and what not.
However this only solves part of the problem because the information is
only partially propagated to slaves (on write operations). Reads will
not affect slaves LFU and LRU counters, so after a failover the eviction
decisions are kinda random until keys start to collect some aging/freq info.

However since new slaves are initially populated via RDB file transfer,
this means that if we spin up a new slave from a master, and perform an
immediate manual failover (for instance in order to upgrade the master),
the slave will have eviction informations to use for some time.

The LFU/LRU info is persisted only if the maxmemory policy is set to one
of the relevant type, even if no actual "maxmemory"  memory limit is
set.

src/rdb.c

@@ -973,6 +973,9 @@ size_t rdbSavedObjectLen(robj *o) {
 int rdbSaveKeyValuePair(rio *rdb, robj *key, robj *val,
                         long long expiretime, long long now)
 {
+    int savelru = server.maxmemory_policy & MAXMEMORY_FLAG_LRU;
+    int savelfu = server.maxmemory_policy & MAXMEMORY_FLAG_LFU;
+
     /* Save the expire time */
     if (expiretime != -1) {
         /* If this key is already expired skip it */
@@ -981,6 +984,26 @@ int rdbSaveKeyValuePair(rio *rdb, robj *key, robj *val,
         if (rdbSaveMillisecondTime(rdb,expiretime) == -1) return -1;
     }
 
+    /* Save the LRU info. */
+    if (savelru) {
+        int idletime = estimateObjectIdleTime(val);
+        idletime /= 1000; /* Using seconds is enough and requires less space.*/
+        if (rdbSaveType(rdb,RDB_OPCODE_IDLE) == -1) return -1;
+        if (rdbSaveLen(rdb,idletime) == -1) return -1;
+    }
+
+    /* Save the LFU info. */
+    if (savelfu) {
+        uint8_t buf[1];
+        buf[0] = LFUDecrAndReturn(val);
+        /* We can encode this in exactly two bytes: the opcode and an 8
+         * bit counter, since the frequency is logarithmic with a 0-255 range.
+         * Note that we do not store the halving time because to reset it
+         * a single time when loading does not affect the frequency much. */
+        if (rdbSaveType(rdb,RDB_OPCODE_FREQ) == -1) return -1;
+        if (rdbWriteRaw(rdb,buf,1) == -1) return -1;
+    }
+
     /* Save type, key, value */
     if (rdbSaveObjectType(rdb,val) == -1) return -1;
     if (rdbSaveStringObject(rdb,key) == -1) return -1;