Set the priority of the crawler thread, not the calling thread, and set the priority to below-normal instead of idle.

git-svn-id: http://tortoisesvn.googlecode.com/svn/trunk@26016 4cfb86a4-2c7a-11df-bd14-c1e331e6ceac
Use the OnOutOfScope() macro to set the background mode for the thread.

git-svn-id: http://tortoisesvn.googlecode.com/svn/trunk@26017 4cfb86a4-2c7a-11df-bd14-c1e331e6ceac