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.

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 */