Support for reducing the number of Capabilities with setNumCapabilities
This patch allows setNumCapabilities to /reduce/ the number of active capabilities as well as increase it. This is particularly tricky to do, because a Capability is a large data structure and ties into the rest of the system in many ways. Trying to clean it all up would be extremely error prone. So instead, the solution is to mark the extra capabilities as "disabled". This has the following consequences: - threads on a disabled capability are migrated away by the scheduler loop - disabled capabilities do not participate in GC (see scheduleDoGC()) - No spark threads are created on this capability (see scheduleActivateSpark()) - We do not attempt to migrate threads *to* a disabled capability (see schedulePushWork()). So a disabled capability should do no work, and does not participate in GC, although it remains alive in other respects. For example, a blocked thread might wake up on a disabled capability, and it will get quickly migrated to a live capability. A disabled capability can still initiate GC if necessary. Indeed, it turns out to be hard to migrate bound threads, so we wait until the next GC to do this (see comments for details).
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