Broadcast holder commitment for currently confirmed funding

A splice's `FundingScope` can only be promoted once a
`ChannelMonitorUpdateStep::RenegotiatedFundingLocked` is applied, or if
the monitor is no longer accepting updates, once the splice transaction
is no longer under reorg risk. Because of this, our current
`FundingScope` may not reflect the latest confirmed state in the chain.
Before making a holder commitment broadcast, we must check which
`FundingScope` is currently confirmed to ensure that it can propogate
throughout the network.

Author: wpaulino