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.
Add a new refresh message to request a keyframe from the pinos server.
pinospay: pass the refresh-request message upstream
pinossink: turn refresh-request messages into events
pinossrc: turn a keyframe event into a refresh-request message