Make `FileLogger` a pure sink

[Frames White]

A sink should basically be a particular type of logger,
with shouldlog(...)=true and min_enabled_level(...) = BELOW_MIN_LEVEL

since it’s child (the thing compose around it),
should be handling those,

and its handle_message is what it should override

[Frames White]

This kind of wants #6 first / at the same time,
as that lets the filtering be done without cost in the 'child' that wraps around it.