sendMessage
Sends a given message using our Stream API. Based on the internal state, we either edit an existing message, or we send a new message, using our API.
Deferred attachments (those without a local file) are resolved on a background thread before the message is handed off to the controller.
It also dismisses any current message actions.
Parameters
message
The message to send.
callback
Invoked when the API call completes.