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This allows us to continue sending/receiving on the session, for example if
the promise will be resolved by other data that the client is going to send.
Specifically, this allows the repl-request-input to work without a deadlock.
It does open the door to interleaved commands/results, which may not be a good
thing overall, but can be restricted separately if necessary (e.g. a flag on
the session).
Matthew Wild