Use new monitor persistence flow in funding_signed handling

Use new monitor persistence flow in funding_signed handling

In the previous commit, we moved all our `ChannelMonitorUpdate`
pipelines to use a new async path via the
`handle_new_monitor_update` macro. This avoids having two message
sending pathways and simply sends messages in the "monitor update
completed" flow, which is shared between sync and async monitor
updates.

Here we reuse the new macro for handling `funding_signed` messages
when doing an initial `ChannelMonitor` persistence. This provides
a similar benefit, simplifying the code a trivial amount, but
importantly allows us to fully remove the original
`handle_monitor_update_res` macro.