Cache sha512 hash and parsed not_before and not_after with X509 cert.

Cache sha512 hash and parsed not_before and not_after with X509 cert.

Replace sha1 hash use with sha512 for certificate comparisons internal
to the library. use the cached sha512 for the validator's verification
cache.

Reduces our recomputation of hashes, and heavy use of time1 time
conversion functions noticed bu claudio@ in rpki client.

ok jsing@ tb@