For SERVER-87128 we need to notify waiting threads that a plan cache key has been added to the plan cache. The best way to do it is to leverage PlanCacheCallbacks.

Unfortunately, PlanCacheCallbacksImpl is implemented in query_plan_cache library while it is used in query_exec. To notify all waiting threads I need to add a dependency defined in query_exec. It seems that the best place for the PlanCacheCallbacksImpl is query_exec, where it is used, and its interface PlanCacheCallbacks should remain in query_plan_cache.

This purely refactoring change would allow to streamline implementation of SERVER-87128 and simplify its code review.