DefaultCorePolicy needs an efficient way to find out when an exclusive thread exits

[hq6]

Today, Arachne does not require the application to inform the runtime when a user-level thread running on an exclusive core exits. However, it is expensive to check on every thread exit for whether the exiting thread was exclusive on a core.
Hence, we lazily check for cores that have been vacated by previous exclusive threads when we need additional cores.
We would like a mechanism for finding out about exclusive thread exit sooner.