vk_scheduler: Drop execution context in favor of views

Instead of passing by copy an execution context through out the whole
Vulkan call hierarchy, use a command buffer view and fence view
approach.

This internally dereferences the command buffer or fence forcing the
user to be unable to use an outdated version of it on normal usage.
It is still possible to keep store an outdated if it is casted to
VKFence& or vk::CommandBuffer.

While changing this file, add an extra parameter for Flush and Finish to
allow releasing the fence from this calls.