Opaque contexts

In Jardin, we are currently exposing the full contents of the structs used as contexts in several places (Jardin Client Async Receive, Jardin Server stream accept and Jardin Server recv). This works perfectly, but makes it difficult future updates of the structs with new members as it would mean recompiling all programs using the library, as public API changed. We should treat the contexts as opaque objects, same way as jardin_client_t and jardin_server_t, and implement getter and setter functions for all members in the struct.