Avoid updating unchanged gossip state

The node might get unchanged gossip states, the state might be just updated after sending a GOSSIP_SYN, then it will get the state that is already up to date. If the heartbeat in the GOSSIP_ACK message is updated, it will unnecessary re-apply the same state again, which could be costly like updating token change.
It's very likely to happen for large cluster when a node startup, as the first gossip message will sync all endpoints tokens, it could take some time (in our case about 200 seconds), during that time, it keeps gossip with other node and get the full token states. Which causes lots of pending gossip tasks.