Using a bounded channel may have blocked the collaboration server
from making progress handling RPC traffic.

There's no need to apply backpressure to calling code within the
same process - suspending a task that is attempting to call `send` has
an even greater memory cost than just buffering a protobuf message.

We do still want a bounded channel for incoming messages, so that
we provide backpressure to noisy peers - blocking their writes as opposed
to allowing them to buffer arbitrarily many messages in our server.

Co-Authored-By: Antonio Scandurra <me@as-cii.com>
Co-Authored-By: Nathan Sobo <nathan@zed.dev>