python: add thread lock around register
    
    In the current implementation, sanlock_register shouldn't block, but we
    should not take any chance, since blocking will hang all threads in the
    caller process.
    
    It is also easier to maintain when we decouple the implementation from
    the python glue layer; we don't want to modify locking in the python
    binding when we we modify the underlying library.
    
    Signed-off-by: Nir Soffer <nsoffer@redhat.com>