Buffer output of running step, transmit pages

[sroidl]

at the moment, every single update of a running step (i.e. every line out output) triggers an update to the UI. The result is that the transmission of large chunks of output (e.g. result of a large output step) takes a long time (i.e. longer than the step itself).

this could be solved by dropping updates and not sending intermediate updates if a newer one is available

[sroidl]

output is updated only once per second, thus limiting the amount of network traffic sent to the client.
this solution should be evaluated

[sroidl]

todo: use async.core mechanisms (throttling channel) instead of half baked own solution