Thread: refactor latch by adding an internal counter that has an associated condition_variable and add doc.

git-svn-id: http://svn.boost.org/svn/boost/trunk@84075 b8fc166d-592f-0410-95f2-cb63ce0dd405
Thread: update externally_locked with assignment.

git-svn-id: http://svn.boost.org/svn/boost/trunk@84076 b8fc166d-592f-0410-95f2-cb63ce0dd405