We filter based on the environment variable `CLN_PLUGIN_LOG`,
defaulting to `info` as that is not as noisy as `debug` or `trace`, at
least libraries will not spam us too heavily.
Changelog-Added cln-plugin: The logs level from cln-plugins can be configured by the `CLN_PLUGIN_LOG` environment variable.
We wrap emitted messages into a JSON-RPC notification envelope and
write them to stdout. We use an indirection over an mpsc channel in
order to avoid deadlocks if we emit logs while holding the writer lock
on stdout.
