Werk #4467: BI aggregation compilation: fixed race condition exception, also forks less processes for compilation

The compiliation process now spawns less worker processes for the compilation, which reduces the overall CPU load.

Furthermore a race condition has been fixed, which was likely to happen when all of these conditions were true:

• there were several dozens BI requests per minute, e.g. from multiple web browsers
• a distributed montitoring setup started to deploy new configurations
• there were lots (>100) of BI aggregations configured

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