Support holding messages after send

[Brett McChesney]

Is your feature request related to a problem? Please describe.
No. This is new functionality.

Describe the solution you'd like
During the simulation I'd like to place a "hold" on the link between two hosts. Any messages sent will remain in the queue while the "hold" is active. At a later time I'd like to remove the "hold", which allows delivery for the queued messages.

This is useful for tests that need to control the ordering of events across multiple hosts.

[Carl Lerche]

Seems like a good feature 👍

[Marc Bowes]

Do you think it's better to hold the messages on the link or in the host? The former seems closer to reality, but requires duplication of the inbox type. I'm inclined towards buffering on the link, but I do the value in simplicity too:

// host.rs
    /// Messages in-flight to the host. Some of these may still be "on the
    /// network".
    inbox: IndexMap<SocketAddr, VecDeque<Envelope>>,

[Brett McChesney]

I was considering adding a "hold" token for any messages added to the queue. The delivery logic can inspect both delivery time and if the "hold" is present.