ocb3: use parallel-capable APIs

[robbie01]

Right now, ocb3 exclusively uses BlockCipherEncrypt::encrypt_block and BlockCipherDecrypt::decrypt_block. Although VAES support is coming in block-ciphers#491, ocb3 won't be able to take advantage of its (2-5x) speedups until encrypt_blocks*/decrypt_blocks* APIs are used.

[newpavlov]

Yes, the implementation should be adopted to the cipher backend API. The current implementation is sub-optimal not only because we do not utilize parallel block processing, but also because we check target features on each encrypt_block call.