it is picked from a palette based on its hash: it is pseudorandom, but
still consistent across different lines and different runs.
The palette is a bit too vivid, but let's stick with it for now.
fixes#10
JSON is actually unfit to send structured log messages, because
date/time is not well supported. So we are using BSON, which supports
those. BSON is, among the dozen serializers available, really a random
choice and might be changed in the future.
