implement transport throttling based on processing state, AMQP is currently missing

* implement a common throttling mechanism based on the current journal state and processing speed (once per second)
* the heuristics probably need a lot of work to make them less jumpy
* there is a new "throttling_allowed" configuration item for transports based on ThrottleableTransport, which overrides whether an input is allowed to be throttled. default is no throttling allowed (like before)

For AMQP we need to decide whether to unsubscribe all the time, or use some backoff mechanism (e.g. throttled for x seconds, unsubscribe, else block)