Update the isValidClientCertificate() method to consider any
client-supplied intermediate certificates. Previously, in order to trust
client certificates issued by an intermediate CA, users would need to
include that intermediate CA's certificate directly in the client_ca
setting. After this change, only the trusted root CA needs to be set: as
long as the client can supply a set of certificates that chain back to
this trusted root, the client's certificate will validate successfully.
Rework the previous CRL checking logic to now consider CRLs for all
issuers in the verified chains.
Update isValidClientCertificate() to also consult the configured
certificate revocation lists. Update existing test cases and add a new
unit test to exercise the revocation support. Restore the skipped
integration test case.
Generate new test certificates and CRLs using a new `go run`-able source
file.
