I'm curious how SERVER-31738 will interact with mongos. If all shards are feature compatibility mode 3.6, I suppose we can expect mongos' isMaster response to include logicalSessionTimeoutMinutes, but what if two shards are 3.6 mode and one shard is not? Does the introduction of a new shard lacking sessions break sessions for the entire cluster? Alternatively, if only one of three shards does not support sessions, is mongos still capable of retrying writes to the shards that do have support?