Log additional details when ignoring first hops

Log additional details when ignoring first hops

Users are often confused when we fail to find a route due to some
requirements on the first hop are not being met. While we now take note
and log such candidates, we still previously required users to check
additional details to figure out why exactly the router refused to route
over a particular first hop.

Here, we add additional TRACE logging, in particular for
`ChannelDetails::next_outbound_htlc_limit_msat` and
`ChannelDetails::next_outbound_htlc_minimum_msat` when they are
relevant.