Decide what to do when the remote closes the last path

[divagant-martian]

Some options:

1. kill the connection immediately.

• PROS: easy to reason about, easy to code, spec compliant
• CONS: The remote might "soon" open a new path, so this is a potentially recoverable situation

2. wait the idle timeout.

• PROS: allows the connection to recover
• CONS: The application might not have an idle timeout. Might make poll_transmit more complex

At time of writing we do 1, which is potentially too harsh

[flub]

I'm also tempted with 3: close after 3 * initial connection PTO (3.1s or there abouts)

[flub]

Let's add some motivation for that one: this would only be for the server side. The client side is not allowed to abandon the last path, so if it does happen to close the last path it must already be trying to open a new path. For that 3*PTO is the generally accepted time for packets to arrive.

This also makes it independent of the configured idle timeout.

[matheus23]

We could decide to go for option (2) for now, which is playing it the most safe way possible (but need to think of something for the no-idle-timeout case), and can later decide to make it more strict.