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.