Optimize the everloving heck out of OpenSSL AES as used by CGO.

Optimize the everloving heck out of OpenSSL AES as used by CGO.

Optimizations:

1. Calling EVP_CryptInit with a cipher returned by
   e.g. EVP_aes_128_ctr() is quite slow, since it needs to look
   up the _actual_ EVP_CIPHER corresponding to the given EVP,
   which involves grabbing locks, doing a search through a
   provider, and so on.  We use EVP_CIPHER_fetch to speed
   that up a lot.

2. There is not in fact any need to EVP_CIPHER_CTX_Reset a
   cipher before calling EVP_CryptInit on it a second time

2. Using an ECB cipher + CRYPTO_ctr128_encrypt was not in fact
   the most efficient way to implement a counter mode with an
   adjustable IV.  Instead, the fastest way seems to be:
     - Set the IV manually
     - Ensure that we are always aligned to block boundary
       when we do so.