channels can become out-of-sync

[ssfrr]

Because I'm using a separate ringbuffer for each channel, it's possible for channels to get de-synced if the callback happens to hit while the read or write is interacting with the ringbuffers. Fix: use a single ringbuffer in each direction and have the reader on each side only act when there's a full block

[ssfrr]

fixed by 3cfbaac

[ssfrr]

Incidentally I didn't use this approach, as it introduces a dependence on the Julia side to know the underlying blocksize. Instead I keep separate per-channel ringbuffers but do a pass through them all before any reads to find the smallest number of samples available, and then read that many from all channels.