* a vm_cache now maintains a list of its "consumer" caches.
* introduced to new functions that add/remove consumer to a cache (instead
of only maintaining the vm_cache::source field).
* fixed the incorrect reference counting when doing copy-on-write; we kept
one ref too many of the lower cache.
* minor cleanup.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@19035 a95241bf-73f2-0310-859d-f6bbb57e9c96
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