conference: make single transfer to main memory

While in a conference, the video split added a transfer to
main memory for each of the participants. This makes the sinkClient
a observable and the participants sinks are attached to the main
conference sink. The main sink has its frame transferred and the
subsequent observers do not need to repeat the process.

GitLab: #709
Change-Id: I1f4ea9460c052a100b4809101c35d196cd79acbe