Fix crash and deadlocks in the support for recursive logging #7127
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The code has been slightly refactored to remove an incorrect usage
of std::futures in a queue, where it was possible to try to wait on the
same std::future twice, which would lead to a crash.
Even after fixing the crash, it was possible that threads dealing
with recursive logging would change order and wait on a std::future
of another thread, causing a deadlock.
Finally the deferred log relaying wasn't always used when it should've,
due to a confusion between boolean values.
A enum class has been used instead to be more expressive about
the relaying mode asked.
Fixes #6928