Implement MaxApplications enforcement

Queues have an option to set the MaxApplications that can run in a queue. There is currently no code in the scheduler that checks this setting.

As a new feature we should add the enforcement for this setting:

• enforce the setting on a leaf queue
• enforce the setting on a parent, the apps running in a parent queue is defined as the sum of all the apps running in all leaf queues of the parent.

As a side note from a config check: we need to make sure that the parent setting cannot be lower than any of the child queues it has. We must not enforce that the parent setting must be larger than sum of all leafs.