qps/rate limit & reduce expire seek range & expire-key hash

[piaoairy219]

1 limit namespace&command qps/rate in view of all the titan server 2 reduce expire seek range to avoid rocksdb tomstone problem 3 hash expire-key to 256 prefix to improving the expire handling speed and prevent expire-key writing focus on single node. 4 handle empty/illegal commands 5 if connection has been closed by the client, drop left command to process 6 limit max-connection