While I'd hoped to be able to make the socket client less weird, I
think that this is a nice middle ground in terms of improving
readability and removing the vestigal components without breaking
anything or radically changing the underlying assumptions.
In the future we'd want to have requests be identified by a request
ID, and then we could drop the request tracking logic in the client
entirely, and this is protocol breaking. The alternatives aren't
substantively different than the current implementation.