prov/sockets: add per domain connection map for sockets level connection.

Sockets provider update.
- Add per domain connection map to manage sockets level connection establishment.
- Update fi_av APIs implementation in sockets provider accordingly.
The returned address information doesn't seemed to be used and isn't freed.  The socket call below uses the ai_family and ai_socktype returned by the call, but since those are provided as hard-coded hints, the returned values are known.