Remove hash negotiation and force sha-256

[pmeenan]

This removes the hashes negotiation and changes the Available-Dictionary request header to explicitly use sha-256 as discussed in #2631. If sha-256 becomes insufficient, the hash negotiation can then be added (or different mechanisms can be used).

[martinthomson]

WFM, though I would also add a paragraph somewhere about what you might do to upgrade SHA-256 if there was ever cause to do so (and maybe why that is unlikely, as we are not relying on any of the properties of the function that are security-relevant).

[pmeenan]

WFM, though I would also add a paragraph somewhere about what you might do to upgrade SHA-256 if there was ever cause to do so (and maybe why that is unlikely, as we are not relying on any of the properties of the function that are security-relevant).

Thanks. I added a paragraph to the security section about what the hash is used for and what it would look like if it was decided that SHA-256 needed to be replaced.