Re-broadcast `channel_announcement`s every six blocks for a week

Re-broadcast `channel_announcement`s every six blocks for a week

When we first get a public channel confirmed at six blocks, we
broadcast a `channel_announcement` once and then move on. As long
as it makes it into our local network graph that should be okay, as
we should send peers our network graph contents as they seek to
sync, however its possible an ill-timed shutdown could cause this
to fail, and relying on peers to do a full historical sync from us
may delay `channel_announcement` propagation.

Instead, here, we re-broadcast our `channel_announcement`s every
six blocks for a week, which should be way more than robust enough
to get them properly across the P2P network.

Fixes #2418