Limit number of pipelines per pipeliner

[turicas]

Currently pipeliner is greedy: as soon as router announces a new pipeline, it gets this pipeline to take care of.
This approach, for a first point of view, can lead to a greater throughput, but if we analyse it better, it is not a good thing to do, because:

• Pipeliner's process will stop with MemoryError exception as soon as there are so many pipelines to use the maximum amount of memory allowed by the operating system;
• As the list of pipelines in Pipeliner will be huge, it'll slow down Pipeliner's operation and can lead in delays to start new jobs, send messages of 'finished pipelines' etc.
• We (maybe?) can have problems with huge queues on ZeroMQ (think in 1M pipelines finishing -- the amount of used memory of this queue and the time Pipeliner will 'stop' doing things just to process it).

So, the new approach should be like Broker's: limit the number of pipelines that Pipeliner takes care at a time. This number can be fixed or dynamic (dynamic is better, I think) and probably will be based on:

• Number of workers in the cluster
• Rate of finished pipelines