Do not use unbounded channels in online audio processing

[orottier]

As mentioned in #297:

I also consider using unbounded channels for this purpose as inappropriate. For any for real world use case bounded channels are recommended. Moreover the receiver side of theses crossbeam channels cannot be used safely in a real-time context AFAIK.

The crossbeam channels seem to have serious unresolved issues: crossbeam-rs/crossbeam#821

The AudioContext (as opposed to the OfflineAudioContext) should only use bounded channels.
It is probably okay for the control thread to block on a full channel.
It is probably okay for the render thread to drop data when a channel is full (we use them for events and metrics).