Thread: update the externally_locked_stream interface.

git-svn-id: http://svn.boost.org/svn/boost/trunk@84109 b8fc166d-592f-0410-95f2-cb63ce0dd405
Thread: update externally_locked.

git-svn-id: http://svn.boost.org/svn/boost/trunk@84110 b8fc166d-592f-0410-95f2-cb63ce0dd405