imjournal: add ratelimiting capability

The original imjournal code did not support ratelimiting at all. We
now have our own ratelimiter. This can mitigate against journal
database corruption, when the journal re-sends old data. This is a
current bug in systemd journal, but we won't outrule this to happen
in the future again. So it is better to have a safeguard in place.
By default, we permit 20,000 messages witin 10 minutes. This may
be a bit restrictive, but given the risk potential it seems reasonable.
Users requiring larger traffic flows can always adjust the value.