fix(iroh-net): split packets on send

[rklaehn]

this seems to be the safest way to fix possible GSO/GRO issues on all platforms (Linux and ???) that support them.

Description

This is a fix for the possible issues described in #1357 that does the simplest possible thing - just split up the packets before sending them to the derper.

Note that we still get GRO/GSO when we have a direct connection. And since the stream to the derper is a byte stream, unless we flush after each send this should still be rather efficient. The only downside is that the derper has to deal with more individual packets.

Notes & open questions

Does this fix the issue?

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.

[rklaehn]

@Arqu once you have a way to test things, can you check if this solves the issue?

[flub]

I think generally this approach is probably the right call for fixing this. we need to do some GSO-like thing properly on the derper channel anyway later.

[rklaehn]

I did the following to validate this:

• log in to a linux box
• enforce derper use
  
• configure a local derper
• run a local derper
• try with split_packets being a noop - not splitting => transfer does not work
• try with split_packets as in this PR - splitting on segment_size => transfer does work

If things are messed up, you get messages like this:

2023-08-21T11:49:53.757198Z TRACE quinn_proto::connection: decryption failed with packet number 5
2023-08-21T11:49:53.757211Z DEBUG quinn_proto::connection: failed to authenticate packet