The legacy MongoClient defaults the heartbeat socket settings to a 20 sec connect timeout and a 20 second read timeout.

MongoClientSettings should similarly ensure that the default timeouts are not infinite.

Given the SDAM spec it's possible that MongoClientSettings shouldn't even expose heartbeat socket settings, instead just using the normal socket settings connect timeout. Have to consider backwards compatibility with the existing async MongoClientSettings, and weight that against what API we want to offer going forward.