Implement the client introduction point establishment process and logic for proposal 224. * Pick IP from descriptor. * Use IP failure cache. * Open introduction circuit. * Setup circuit identifier on intro circuit. * Generate/Send INTRODUCE1 cell. * Handle INTRODUCE_ACK cell. In legacy code, see `rend_client_send_introduction()` and `rend_client_introduction_acked()` in **src/or/rendclient.c**