Start a preconnection to msg.
Once the connection is done, it will remain in idle state so that it can be reused by future requests. If there's already an idle
connection for the given msg host, the operation finishes successfully without creating a new connection. If a new request
for the given msg host is made while the preconnect is still ongoing, the request will take the ownership of the connection
and the preconnect operation will finish successfully (if there's a connection error it will be handled by the request).
The operation finishes when the connection is done or an error occurred.
| this |
a Session |
| msg |
a Message |
| io_priority |
the I/O priority of the request |
| cancellable |
a Cancellable |
| callback |
the callback to invoke when the operation finishes |
| user_data |
data for |