activate the AssertHeld() logic of Mutex and Spin classes.  Used in debug builds only.
Correct error path for code changed to support simultaneous compactions (error found by code in issue #100).  Populate Mutex.AssertHeld() code ... Google, why left empty?
Merge pull request #125 from basho/mv-mutex-assert

activate the AssertHeld() logic of Mutex and Spin classes. And address issue #100