Merge pull request #2498 from birarda/master

fix thread contention and reinstate distance based avatar-mixer sending
add a method to Agent for last read frame loudness
Merge pull request #2510 from birarda/agent-loudness

add a method to Agent for last read frame loudness
Merge branch 'master' of https://github.com/highfidelity/hifi
wait on broadcastThread before destroying AvatarMixer, closes #2499