TS-2893: improved support for ECDSA certificates

Add the ability to deliver different certificates to the client
depending on the cipher list advertised by the client.

The directive is to support loading multiple PEM certificate chains
with different algorithms at a time, each PEM file may include
multiple certificates (e.g. cert1&key1 + intermediate cert2 &
key2+...). After a few more thinking, I realized it should be OK
to reuse ssl_cert_name to do that.

To simplify the SSL certificate configurations, perhaps we can
obsolete ssl_ca_name and proxy.config.ssl.server.cert_chain.filename
gradually.