Broadcast: Allow write guards to be disarmed before drop

child issue tracker for #142. Implement it for broadcast write guards.

This child issue points to parent issue #142 and names broadcast write guards, but it provides no files or tests. Read #142 first, then locate the broadcast write-guard implementation in this Rust repository. Done means the behavior requested by #142 is implemented for broadcast guards and the relevant checks pass.

索引モデルが issue の本文から書いたものです。