This was just an oversight when route calculation was split up into
parts - it makes no sense for get_route to require that we have a
full route message handler, only a network graph (which can always
be accessed from a NetGraphMsgHandler anyway).
This caused a bunch of cascading changes, including
passing loggers down to Channels in function calls
rather than having each Channel have a pointer to the
ChannelManager's Logger (which was a circular reference).
Other structs that the Channel had passed its Logger to also
had their loggers removed. Other newly unused Loggers were
also removed, especially when keeping them would've caused
a bunch of extra test changes to be necessary, e.g. with
the ChainWatchInterfaceUtil's Logger.
