implement hot redundancy concept for scenario critical tasks (QOS KPI#13)

[lukasz-zet]

Scenario critical tasks, as defined by Quality of Service (QoS) parameters, will be guaranteed to have less than ≤ 10 ms downtime upon (simulated) failure of any system component, thanks to hot stand-by redundant executors.

[ccicconetti]

I guess this issue depends on #96.