Prop 313: 5. Collect IPv6 Connection Stats on Relays

We propose that relays (but not bridges) collect IPv6 connection statistics.

Bridges refuse to collect the existing ConnDirectionStatistics, so we do not believe it is safe to collect the smaller IPv6 totals on bridges.

To minimise development and testing effort, we propose re-using the existing "bidi" code in rephist.c. (This code may require some refactoring, because the "bidi" totals are globals, rather than a struct.)

(We might also want to move this code into separate relay-only code and header files, because it is relay-specific.)

In particular, tor currently counts these connection statistics:

• below threshold,
• mostly read,
• mostly written, and
• both read and written.

We propose adding IPv6 variants of all these statistics. (The IPv4 statistics can be calculated by subtracting the IPv6 statistics from the existing total connection statistics.)

See proposal 313, section 5: https://gitweb.torproject.org/torspec.git/tree/proposals/313-relay-ipv6-stats.txt#n144