This adds SenderQueue and a wrapper of messages to
send back to clients.
Currently no threading takes place, but the messages
are pumped through the queue nonetheless.
Change-Id: Id9997539c0a2a351cbf406f649c268dd3643e88e
Reviewed-on: https://gerrit.libreoffice.org/31883
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>
Tested-by: Ashod Nakashian <ashnakash@gmail.com>