`BaseAddressCache._gen_cache_key` is too fast

[todofixthis]

BaseAddressCache._gen_cache_key uses a single iteration of SHA-256, which is too fast to be secure against brute-force / rainbow table attacks.

An algo like PBKDF2 or bcrypt would be far more secure (although the salt would need to be constant, since the resulting hashes need to function as lookup keys).

See https://docs.python.org/3/library/hashlib.html#hashlib.pbkdf2_hmac for more info.

Interestingly, Python 3.6 introduces an scrypt implementation into stdlib; might be interesting to allow the user to specify which algo they want to use when initializing the cache.

[todofixthis]

Note: This is only relevant to the pycurl implementation. If we were to modify the address cache to use a secure hash algo, it would actually take longer to make the lookup key to check the cache than it would for the C extension to regenerate the address!

I suspect that the long-term solution here is to get the C extension working in Python 2 and then make it part of the library — no pycurl at all, and no need for address caches.

[todofixthis]

Just to be safe, I'm going to remove address caches for now. We can always put them back in later if there is a genuine need for them (and we can come up with a secure implementation).