Make it possible to pass custom memory to the builder so that it can
construct packets directly into specified memory and avoid allocs.
Remove GError and GSocketControlMessage in the buffer API to make it
possible to use other (rt-safe) API later.
Only go to READY in setup_pipeline, this is enough to get the caps. We
don't want to go to PAUSED yet because we don't want to negotiate a
format.
Mark the source busy after we configured the capsfilter so that we can
let the source negotiate correctly.
Send the base-time to the client so that it can calibrarte the clock.
Wait until we are completely negotiated before completing the state
change to paused. This makes it possible to provide a clock and decide
if we are live or not.