python: use wrapped notmuch_*_t types instead of raw pointers

python: use wrapped notmuch_*_t types instead of raw pointers

Now that types are checked correctly, we also need to make sure that all the
arguments actually are instances of these types. Otherwise the function calls
will fail and raise an exception similar to this one:
   ctypes.ArgumentError: argument 3: <type 'exceptions.TypeError'>: expected
   LP_LP_NotmuchMessageS instance instead of pointer to c_void_p