Improve event infrastructure

[kevzhao2]

Our event infrastructure should have the following characteristics:

• Ensure server QoS. This can be done by adding budgets to event types, and ensuring that we don't go too far over budget when running the event handlers.
• Asynchronous event handlers. These are event handlers which are run after the synchronous event handlers, and they optionally can be non-blocking, meaning that the event "ends" even before these event handlers are finished running.

[kevzhao2]

Asynchronous event handlers have been implemented. Still designing a solution for ensuring QoS.