js: avoid blocking the event loop

[zenhack]

...by using spsc, an unbounded queue, instead of a channel for messages.

This makes use of spsc's .Close() method, introduced in https://github.com/capnproto/go-capnproto2/pull/449

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In principle, without this you could deadlock the event loop by creating a websocket connection and then never actually calling .Decode(), though this is unlikely in normal use.

[zenhack]

Can't merge this until the corresponding go-capnp pr is merged, so parking it here for now.