Fix regression of admin secure URL due to new hash algo

Admin secure URLs use encryptor hash to generate and validate secret keys in url.
Since native API auto generates salt, they never validate.

This branch makes the salt handling more backwards compatible so that admin can work.

However, PHP native hash algo is really slow for the excessive number of hashing done for secure URLs. Added fastHash() method to hash using legacy algorithms, and rewrote adminhtml/url model to use it.
Since secure URLs are very transient, I think it's ok to use a fash hash for them.