implement new ratelimiting mode for imjournal

and make imjournal use it.

The new mode is needed, as imjournal uses journal's timestamp
as message generation time (which otherwise is when the message
entered the system, and the ratelimiter uses this as current
timestamp in order to save performance).

It is debatable if imjournal is doing the right thing here. But
it doesn't feel totally wrong. So let's safe that debate for
later ;)