Explicitly set next extension id.
Correctly encode packets with multiple extensions.
Replace duplicated calculation with variable.
Ignore unknown extensions instead of failing.
Partially rewrite `flush_incoming_buffer`.
Ensure we can always dump the remaining data.
Merge branch 'rewrite'
Remove unnecessary parameter from `UtpSocket.send_to`.
Merge branch 'api_change'
Use getter for property.
Clean up.
Remove Clone implementation for UtpSocket.
Fix bug where send window would stop being flushed.

It happened whenever the sequence number rolled over to 0, making it impossible
to pop the packets that preceded the latest acknowledged because their sequence
number was larger.
Use primitive congestion control for now.
Merge branch 'fix_rollover_seqnr'