Move from using CCS (before: established) to just doing certificate
validation at the end of the connection.
This is (again) more robust in the case of aborted connection. I am
moving this into a hook because of the complexity of the
end-of-connection handling for SSL.
This should probably be extended to not just handle SSL validation, but
all other logging constructs that are currently called in _established.
This triggers a re-validation of the SCT when the certificate validation
status changed. This means that a SCT will show as valid if the
certificate chain could be validated and can now be used to determine
the correct issuer key hash.
This also rewrites the certificate validation script (which we need for
this) slightly.
This could need a bit of caching, but should generally work very
reliably.
