Problem is that io_run_until_read can finish early while paused, because
getting the response and first data frames can happen in the same
nghttp2_session_mem_recv() call, so when the message is paused we have
already progressed to reading state. Returning NGHTTP2_ERR_PAUSE from
callbacks might be problematic too, because it will block other streams
not paused. Since we will read the body in any case when unpaused,
it's ok to let the read continue while paused, but without finishing
io_run_until_read() until it's unpaused.