Added a ChangeLog entry.

src/lib/config/command_mgr.cc
    CommandMgrImpl::openCommandSocket() - now attempts to open and
    lock a lock file whose name is derived from the control socket
    name.  If the lock cannot be established, it is assumed that the
    socket is in use.

    CommandMgr::closeCommandSocket() - removes the lock file after
    the socket file is removed.

src/lib/config/tests/command_mgr_unittests.cc
    TEST_F(CommandMgrTest, exclusiveOpen) - new unit test