Remove direct reference to conn->fd and use the connection abstraction

[madolson]

This pierces the connection abstraction, which we don't want to do. Connections are supposed to be opaque to us. This is also documented clearly enough to understand why we are shutting down the repl channel in the TLS case. Maybe add a flag to connShutdown here.

Originally posted by @madolson in https://github.com/valkey-io/valkey/pull/837#discussion_r1714214260