librbd: add data encryptor/decryptor using openssl

librbd: add data encryptor/decryptor using openssl

This commit adds the first CryptoInterface implementation
which translates bufferlist encryption to the standard api of crypto libraries,
i.e. using block ciphers operating on C-style arrays.

It defines an abstract DataCryptor interface to allow different crypto libraries implementations.
With this commit, we add an OpenSSL based implementation.
Lastly, we implement a context-pool wrapper implementation to optimize performance.

Signed-off-by: Or Ozeri <oro@il.ibm.com>