Clears the open-step stack and resets the internal id counter. Mainly
useful for tests and interactive/knitr re-runs where a previous run may
have left the stack non-empty (e.g. after an uncaught error with no
with_logging() wrapper).
Details
Registered sinks are deliberately not cleared: they are configuration
rather than run state, so a file sink set up once keeps recording across
resets. Take one off with logtree_sink_remove().