crypto: use EVP for AES-CBC

This enables the use of hardware acceleration for AES crypto on newer Intel CPUs (AES-NI), among other platforms. These acceleration features are only available when using EVP (you can compare the results of "openssl speed aes-256-cbc" to "openssl speed -evp aes-256-cbc" to see if this will improve a particular machine's throughput).

On a CPU with AES-NI this can give a pretty significant speedup. I've seen a ~4x reduction in CPU time used by a toy app that just shoves data between two SSL sockets, but ymmv, naturally.