Fix bypass buffers.
Combines a few changes to try to fix bypass buffers.
Besides refactors, this:
- Correctly calculates the
next_tstart
, i.e. the time that the current snr buffers should end at. - If (somehow) postcoh fails to send a buffer, the snr pads will now be flushed by the corresponding amount. I added an assertion in case this happens to ensure we've actually got enough data to flush while leaving enough leftover to run on.