message: Remove bad assert in TxBuffer::mark_as_acked()
  It is not true that highest_offset >= offset+len, because we unmark ranges
  that were sent but need to be retransmitted (b/c lost.) If this happens and
  then an ack comes in for the range, that is ok.
  fixes
diff: @@ -335,8 +335,6 @@ impl TxBuffer {
  }
  pub fn mark_as_acked(&mut self, offset: u64, len: usize) {
  - assert!(self.ranges.highest_offset() >= offset + len as u64);
  -
  self.ranges.mark_range(offset, len, RangeState::Acked);
  // We can drop contig acked range from the buffer
  