Members include ReplSetMetadata in heartbeat requests and replies, and oplog query replies, in order to gossip their replication state. Members include their configVersion when they send ReplSetMetadata today, but configVersion alone isn't enough for members to be certain they have the same replica set config.

In 4.4, for the Safe Reconfig project, we added a term field to replica set configs. Add this term as a "configTerm" field to ReplSetMetadata. If two nodes have the same configVersion and configTerm, then they certainly have the same config.

There is no specific use for configTerm in ReplSetMetadata now, but I anticipate it will be useful if we add it. Adding it now in 4.4 will make it usable in 4.6 without upgrade/downgrade fuss.

(We thought of this during the SERVER-46667 investigation.)