This can potentially provide a large amount of savings,
because CA certificates can be quite large.

Based on previous work by Mini Hawthorne.

Based on previous work by Mini Hawthorne.

EVP_KEY objects are a reference-counted container for key material, shallow
copies and OpenSSL stack management aren't needed as with certificates.

Based on previous work by Mini Hawthorne.

Certificate chains are now loaded once.

The certificate cache provides each chain as a unique stack of reference
counted elements.  This shallow copy is required because OpenSSL stacks
aren't reference counted.

Based on previous work by Mini Hawthorne.

Added ngx_openssl_cache_module, which indexes a type-aware object cache.
It maps an id to a unique instance, and provides references to it, which
are dropped when the cycle's pool is destroyed.

The cache will be used in subsequent patches.

Based on previous work by Mini Hawthorne.

Instead of cross-linking the objects using exdata, pointers to configured
certificates are now stored in ngx_ssl_t, and OCSP staples are now accessed
with rbtree in it.  This allows sharing these objects between SSL contexts.

Based on previous work by Mini Hawthorne.

Starting from TLSv1.1 (as seen since draft-ietf-tls-rfc2246-bis-00),
the "certificate_authorities" field grammar of the CertificateRequest
message was redone to allow no distinguished names.  In TLSv1.3, with
the restructured CertificateRequest message, this can be similarly
done by optionally including the "certificate_authorities" extension.
This allows to avoid sending DNs at all.

In practice, aside from published TLS specifications, all supported
SSL/TLS libraries allow to request client certificates with an empty
DN list for any protocol version.  For instance, when operating in
TLSv1, this results in sending the "certificate_authorities" list as
a zero-length vector, which corresponds to the TLSv1.1 specification.
Such behaviour goes back to SSLeay.

The change relaxes the requirement to specify at least one trusted CA
certificate in the ssl_client_certificate directive, which resulted in
sending DNs of these certificates (closes #142).  Instead, all trusted
CA certificates can be specified now using the ssl_trusted_certificate
directive if needed.  A notable difference that certificates specified
in ssl_trusted_certificate are always loaded remains (see 3648ba7db).

Co-authored-by: Praveen Chaudhary <praveenc@nvidia.com>

The directive allows to pass upstream response trailers to client.