Use util methods in `Peer` to decide when to forward

Use util methods in `Peer` to decide when to forward

This consolidates our various checks on peer buffer space into the
`Peer` impl itself, making the thresholds at which we stop taking
various actions on a peer more readable as a whole.

This commit was primarily authored by `Valentine Wallace
<vwallace@protonmail.com>` with some amendments by `Matt Corallo
<git@bluematt.me>`.