SSL layer - deprecate old per-connection credentials API, introduce new top-level configuration domain

In the first two releases of the SSL layer, the model only provided a per-connection object (pn_ssl_t). Thus, all configuration had to be applied each time an SSL connection was created.

As new capabilities were added to SSL, this model has proven to be inadequate.

With session resume, a new top-level object was introduced: pn_ssl_domain_t. This object becomes a "factory" for pn_ssl_t connections. Common configuration, such as credentials and CA database, are configured at the pn_ssl_domain_t level, and are adopted by each connection (pn_ssl_t) that is created.

This model not only reduces the configuration actions need by the app, but results in a cleaner SSL implementation since the underlying SSL libraries (OpenSSL and Java's SSL) use a very similar model.

More detail of the new api can be seen in PROTON-136.

The problem now become support of the old api. Behavior becomes a bit vague if we allow credential, etc, configuration on both the domain and the ssl connection. Given the brief exposure of the existing api, we'd like to drop support for the following existing SSL API methods:

pn_ssl() - constructor
pn_ssl_init()
pn_ssl_set_credentials()
pn_ssl_set_trusted_ca_db()

The corresponding functionality would instead be available via the domain-based api.