#14818: Get MainBuffer as a reference instead of a pointer

daemon/src/audio/alsa/alsalayer.cpp

daemon/src/audio/audiolayer.cpp

daemon/src/audio/audiortp/audio_rtp_record_handler.cpp

daemon/src/audio/pulseaudio/pulselayer.cpp

daemon/src/audio/recordable.cpp

daemon/src/call.cpp

daemon/src/conference.cpp

daemon/src/managerimpl.cpp

daemon/src/managerimpl.h