Reset average ttl when empty databases (#8106)

On FLUSHDB or full sync, reset old average TTL stat.
This Stat is incrementally collected by the master over time when it searches for expired keys. 

(cherry picked from commit b6916ca91c623a892f61cc9d1958c19490eb73ae)

src/db.c

@@ -416,6 +416,9 @@ long long emptyDbGeneric(redisDb *dbarray, int dbnum, int flags, void(callback)(
             dictEmpty(dbarray[j].dict,callback);
             dictEmpty(dbarray[j].expires,callback);
         }
+        /* Because we will start a new database, reset average ttl. */
+        dbarray[j].avg_ttl = 0;
+        dbarray[j].expires_cursor = 0;
     }
 
     /* Post-flush actions */