Generate certficates in a dedicated thread

[paullouisageneau]

Certificates should be generated on a dedicated thread instead of using the threadpool, otherwise it can lead to deadlocks when the threadpool is small.

[KrystianBigaj]

How about doing something like this:

• Give each thread pool task some kind of identifier, or possibly use the promise pointer as an identifier if that is feasible.
• Instead of calling mCertificate.get(), do something like mCertificate.try_dequeue_by_id_and_execute() first, with a better name, obviously.
• Then call mCertificate.get().

Instead of dequeuing the task, a std::atomic_flag::test_and_set approach could also work.

This way, you wouldn't need another thread pool. If the calling thread is about to wait for a task that has not started executing yet, it could dequeue and execute that task on the calling thread itself instead of just blocking and waiting.

I think this should also fix the deadlock.