Make multiple log types configurable, each with its own fliters

At present, only one set of log filters is supported, and it applies to both the stdout log and the journald log.

Instead we probably want to have a [logging] section in our configuration, that allows multiple log types to be configured, each with its own set of filters.

At first we should probably support stdout, journald, and writing data to a file (perhaps with tracing-appender?). It might also be nice to support people who use syslog(), but that's not a v0 requirement.

cc @janimo