Relinquishing primary makes vote request from higher-priority node fail

If only 2 nodes in a replset are alive, for example, one node in a 3-node replset has been down, when the node with a higher priority starts to run election, the other node which is the current primary will detect the higher term, step down and close all connections immediately before voting for the higher-priority node, which leads to an election failure.

We'll tag the connection used by vote request and close other connections when relinquishing primary.