- [API Change] Disable packet sending if full batch cannot be sent
  If lsquic_packets_out_f() cannot send the whole batch, disable
  packet sending until lsquic_engine_send_unsent_packets() is called.
- [BUGFIX] Handle case when STREAM frame does not fit.
- [BUGFIX] Always allow incoming STREAM frames to overlap.  Peers
  may send overlapping STREAM frames even if using versions older
  than Q043.
- Custom header set fixes:
  - set "FIN reached" flag when custom header with FIN flag is
    claimed;
  - do not return custom header set for a reset stream.