Circuit reactor should poll input even if chan_sender is blocked

If the chan_sender is blocked, the circuit reactor currently stops reading from the application streams and input channel. This was an intentional design choice back when we implemented #1816 (closed), but it's not quite right, as the input channel should be read from regardless of chan_sender readiness (because it shouldn't be possible for input messages to cause us to buffer unboundedly in the chan_sender buffer).