MINIFI-505 Change digest algorithm to SHA-512
MINIFI-505 Change digest algorithm to SHA-512

The reason for this change is that SHA-512 its 50% faster than SHA-256 64-bit platforms that are common nowadays.
https://crypto.stackexchange.com/questions/26336/sha512-faster-than-sha256

This closes #163.

Signed-off-by: Aldrin Piri <8746015474cd01110181ed51735247560fd08de4@apache.org>

apiri/nifi-minifi,apiri/nifi-minifi,apiri/nifi-minifi