Improve WebAudio messaging model

[Manishearth]

While this isn't specced, I was told by @padenot that the ideal messaging model for WebAudio involves one where webaudio commands are batched up in between event loop ticks, and similarly any communication from webaudio to script should happen during these ticks.

To do this the webaudio impl will likely have to be rearchitected to use a message-passing model where the AudioContext is actually a thin wrapper around a sender and some state, and every audio context tick we update the script loop with things like the new currentTime value (etc).