statistics: Properly count all rendezvous cells (avoid undercounting).
tl;dr We were not counting cells flying from the client to the service, but we were counting cells flying from the service to the client. When a rendezvous cell arrives from the client to the RP, the RP forwards it to the service. For this to happen, the cell first passes through command_process_relay_cell() which normally does the statistics counting. However because the `rend_circ` circuit was not flagged with `circuit_carries_hs_traffic_stats` in rend_mid_rendezvous(), the cell is not counted there. Then the cell goes to circuit_receive_relay_cell() which has a special code block based on `rend_splice` specifically for rendezvous cells, and the cell gets directly passed to `rend_circ` via a direct call to circuit_receive_relay_cell(). The cell never passes through command_process_relay_cell() ever again and hence is never counted by our rephist module. The fix here is to flag the `rend_circ` circuit with `circuit_carries_hs_traffic_stats` so that the cell is counted as soon as it hits command_process_relay_cell(). Furthermore we avoid double-counting cells since the special code block of circuit_receive_relay_cell() makes us count rendezvous cells only as they enter the RP and not as they exit it. Fixes #40117.