Resumes HTTP I/O on msg.
Use this to resume after calling pause_message.
If msg is being sent via blocking I/O, this will resume reading or writing immediately. If msg is using
non-blocking I/O, then reading or writing won't resume until you return to the main loop.
This may only be called for asynchronous messages (those sent on a SessionAsync or using queue_message).
| this |
a Session |
| msg |
a Message currently running on this |