Start a new connection process for the given socket.

The only purpose of this callback is to start a process that will handle the socket. It must spawn the process, link and then return the new pid. This function will always be called from inside a supervisor.

This callback can also return two pids. The first pid is the pid of the process that will be supervised. The second pid is the pid of the process that will receive ownership of the socket. This second process must be a child of the first. This form is only available when connection_type is set to supervisor.

If any other value is returned, the supervisor will close the socket and assume no process has been started.

Do not perform any operations in this callback, as this would block the supervisor responsible for starting connection processes and degrade performance severely.