handle a response suspeneded by migration

- All the modification is highly experimental (means dirty).
  One strong assumption is that a migration should take place only
  during either no inflight data in in/out stream or inflight body
  data in out stream. For the latter case, once a migrated connection
  starts and if inflight response exists on its output channel,
  it resumes write_body skipping write_header and finally does
  write_footer.