[IMP] base: prevent SHA-1 collision

SHA-1 is a cryptographic hash function that have weaknesses known since
2005, it has been deprecated by the NIST [1] about 10 years ago in 2011
and Google [2] have been able to perform a collision attack in 2017.

We use SHA-1 as filename for the attachments stored in our filestore.

Although practical attacks still requires quite a lot of computational
resources, it is time to prevent SHA-1 collisions on our filestore.

To prevent such attacks, when uploading a file that has the same SHA-1
as a stored file, we perform a SHA-2 computation on both files to verify
their signature. If their SHA-2 signature is different, an error is
raised.

We have selected the SHA-512 variant of the SHA-2 algorithm because on
64 bits platform, SHA-512 is the fastest SHA-2 variant, it is only ~1.5x
slower than SHA-1. [3]

We have not used SHA-3 because at the moment of writing, it is too slow
(~3x slower than SHA-1) [3] and it is not guaranteed to be available
with the Python 3.5 `hashlib` module.

[1]: https://csrc.nist.gov/projects/hash-functions/nist-policy-on-hash-functions
[2]: https://shattered.io/
[3]: http://bench.cr.yp.to/results-hash.html
[4]: http://www.commitstrip.com/en/2017/02/27/the-sha-1-alternative/

closes odoo/odoo#36430

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>

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