Avoid commitment broadcast upon detected funding spend

Avoid commitment broadcast upon detected funding spend

There's no need to broadcast our local commitment transaction if we've
already seen a confirmed one as it'll be immediately rejected as a
duplicate/conflict.

This will also help prevent dispatching spurious events for bumping
commitment and HTLC transactions through anchor outputs (once
implemented in future work) and the dispatch for said events follows the
same flow as our usual commitment broadcast.