Make sending the payloads async
Fixes the bug described in !359 (merged). Since we made sending packets sequential in !90 (merged) (including the payload part of the packet) we could be blocking the queue for up to 10 seconds if the other end didn't fetch our payload.
This makes the payload part async by default but keeps the option to make it sync, since we want that behavior in CompositeUploadFileJob.
This fixes KDE Connect being irresponsive for a few tens of seconds after connecting if trying to send payloads (eg: notification icons) that were not being downloaded by the other end.