ethereum/utp shutdown behavior

What do we want to do with our shutdown behavior

Currently if all of your outgoing buffer is sent we just instantly send a fin.

Compared to libutp where it doesn't try to send a fin packet, it will send keep alive ack packets and lets the user decide, when we are finished

Start by comparing the project's shutdown behavior with libutp's behavior described in the issue. Determine whether an immediate FIN or keep-alive ACKs with user-controlled completion is intended, then document the decision and verify the resulting shutdown behavior.

Written by the indexing model from the issue text.