Feat/realtime trigger thread

[loicmathieu]

This PR will dispatch the evaluation of realtime triggers to a virtual thread pool. To protect against too many triggers running, it can be limited when launching the Worker via --realtime-trigger-thread, when the pool is exhausted the realtime trigger fail. By default the pool is unbounded.

For monitoring purpose, 2 new metrics has been created:

• worker.realtimetrigger.thread: the number of max threads to start for realtime triggers
• worker.realtimetrigger.running: the number of realtime triggers running in this worker