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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.
5 jobs for bug40117_035 in 23 minutes and 56 seconds (queued for 1 minute and 10 seconds)
latest
Status Job ID Name Coverage
  Test
passed #2117
debian-distcheck

00:06:02

passed #2118
debian-docs

00:07:24

passed #2116
debian-hardened

00:13:57

passed #2119
debian-integration

00:07:15

passed #2115
debian-minimal

00:07:38