Make descriptor_cache usable from parallel threads

[johannesugb]

The avk::descriptor_cache shall be usable from parallel threads. This is partly already prepared by the means of different descriptor_pools being used from different threads (see: std::unordered_map<std::thread::id, std::vector<std::weak_ptr<descriptor_pool>>> mDescriptorPools; and avk::descriptor_cache::get_descriptor_pool_for_layouts).

What needs to be added is a std::mutex that synchronizes access.

• Add std::mutex to class descriptor_cache
• Use it in member methods that modify members or access members that could be modified from parallel threads (best synchronize using std::scoped_lock<std::mutex>)

To test this issue, cg-tuwien/Gears-Vk#41 would have to be implemented.

Definition of done:

• std::mutex-based synchronization has been added to avk::descriptor_cache::get_descriptor_pool_for_layouts.
• It has been checked if further synchronization is required in the methods of avk::descriptor_cache and if so, the necessary measures have been taken.
• Usage of the descriptor_cache from parallel threads has been tested (e.g. by using a parallel_invoker)
• All modified functions/methods are well documented and the Contribution Guidelines have been followed.