this fixes bug #227 again (which I recently opened again accidently).
* We actually switched the last consumer's source without having acquired its
lock! This fixes some rare random app crashes as well as potential kernel
crash ("cache to be deleted still has consumers").
* Some more comments to explain why things are done and can be done the way they
are done :-)
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@19878 a95241bf-73f2-0310-859d-f6bbb57e9c96