The old CoreSuspender requires processing from Core::Update to allow
commands execute. But that causes issues if Core::Shutdown wants
quarentee cleanup order with std:🧵:join. Fixing shutdown ordering
adds too many branches to already fairly complex code.
I decided to try to refactor CoreSuspender to use simpler locking
locking using a std::recusive_muted as primary synchronization
primitive.
To help control when Core::Update unlocks the primary mutex there is
std::contition_variable_any and std::atomic<size_t> queue lenght
counter.
The last state variable is std::atomic<std:🧵:id> that is used to
keep track of owner thread for Core::IsSuspended query.
This should be merged only just after a release to make sure that it
gets maximum testing in develop branch before next release.
Fixes#1066
f6b0ac7819