LFU: do some changes about LFU to find hotkeys

Firstly, use access time to replace the decreas time of LFU.
For function LFUDecrAndReturn,
it should only try to get decremented counter,
not update LFU fields, we will update it in an explicit way.
And we will times halve the counter according to the times of
elapsed time than server.lfu_decay_time.
Everytime a key is accessed, we should update the LFU
including update access time, and increment the counter after
call function LFUDecrAndReturn.
If a key is overwritten, the LFU should be also updated.
Then we can use `OBJECT freq` command to get a key's frequence,
and LFUDecrAndReturn should be called in `OBJECT freq` command
in case of the key has not been accessed for a long time,
because we update the access time only when the key is read or
overwritten.

src/object.c

@@ -1050,10 +1050,14 @@ void objectCommand(client *c) {
         if ((o = objectCommandLookupOrReply(c,c->argv[2],shared.nullbulk))
                 == NULL) return;
         if (!(server.maxmemory_policy & MAXMEMORY_FLAG_LFU)) {
-            addReplyError(c,"A non-LFU maxmemory policy is selected, access frequency not tracked. Please note that when switching between policies at runtime LRU and LFU data will take some time to adjust.");
+            addReplyError(c,"An LFU maxmemory policy is not selected, access frequency not tracked. Please note that when switching between policies at runtime LRU and LFU data will take some time to adjust.");
             return;
         }
-        addReplyLongLong(c,o->lru&255);
+        /* LFUDecrAndReturn should be called
+         * in case of the key has not been accessed for a long time,
+         * because we update the access time only
+         * when the key is read or overwritten. */
+        addReplyLongLong(c,LFUDecrAndReturn(o));
     } else {
         addReplyErrorFormat(c, "Unknown subcommand or wrong number of arguments for '%s'. Try OBJECT help",
             (char *)c->argv[1]->ptr);