This commit fixes a bug where the same encryption key used for two stanzas in
the access.conf file would result in access requests that matched the second
stanza to always be treated as a replay attack.  This has been fixed for
the fwknop-2.0.1 release, and was reported by Andy Rowland.  Now the fwknopd
server computes the SHA256 digest of raw incoming payload data before
decryption, and compares this against all previous hashes.  Previous to this
commit, fwknopd would add a new hash to the replay digest list right after
the first access.conf stanza match, so when SPA packet data matched the
second access.conf stanza a matching replay digest would already be there.