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.
This sock_av_lookup_addr is not the implementation of fi_av_lookup. It's a utility function that will be called by progress engine to find a socket connection based on given av and fi_addr_t. When the key is NULL, it means this is the first time to send to(recv from) this remote address. Then the sock_conn_map_set_key function is called to find the index in the connection map for this remote address and store this index to the key_table belong to the given av.