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This paper has been withdrawn by the author due to a crucial argument error at p.10.

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This paper has been withdrawn by the author, due an error in the proof of Proposion 2.13.

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This paper has been withdrawn by the authors, since it has been merged with Part I (ID 0802.3570)

This paper has been withdrawn by the author due to errors.

This paper has been withdrawn by the authors.

The paper has been withdrawn by the author.

This paper has been withdrawn by the author due to a crucial error in the last lines in the proof of Lemma 3.3.

The paper has been withdrawn.

This paper has been withdrawn by the authors until some changes are made.

This paper has been withdrawn by the author. A revised and expanded version is gr-qc/9907028 (Phys.Rev. D60 (1999) 104043).

This paper has been withdrawn by the author(s) in the light of several other works available and due to a misunderstanding in the authorships.

This paper is a revised version of a previously posted paper in arxiv. The authors posted it as a new submission by mistake. The latest version of the paper can be found at arXiv:math-ph/0512003v2

This paper has been withdrawn by the author due to serious error found in main argument.

This paper has been withdrawn by the author due to an error in the main proof (thanks to Carlos D'Andrea)

This paper has been withdrawn by the author because there are some typos in proofs.