Signal when pruning should kick in

[michaelsauter]

Without sending on the channel, pruning will never happen 🤦

Fixes #557.

Tasks:

• [x] Updated design documents in docs/design directory or not applicable
• [x] Updated user-facing documentation in docs directory or not applicable
• [x] Ran tests (e.g. make test) or not applicable
• [x] Updated changelog or not applicable