Avoid early return upon confirmation of channel funding

Avoid early return upon confirmation of channel funding

This early return is only possible if the channel requires a single
confirmation, allowing a `channel_ready` message to go out. This can be
problematic though if a commitment transaction (specifically from the
counterparty, as the channel would be immediately closed if a local
commitment is broadcast) also confirms within the same block. The
`ChannelMonitor` will detect both, but it won't inform the
`ChannelManager` at all. Luckily, while the channel still is considered
open to the `ChannelManager`, the `ChannelMonitor` will reject any
further updates to the channel state.