Adjust "large number of connections to other relays" warnings

In legacy/trac#33880 (moved), we adjusted the limits MIN_RELAY_CONNECTIONS_TO_WARN, and the default number of connections allowed per relay.

In legacy/trac#33048 (moved), we will allow relays to extend via IPv6 as well as IPv4. So we should make these changes:

warn if any relay has more than 4 connections (that is, more than 2 sides multiplied by 2 IP addresses, if there is disagreement over canonical connections).
ignore 2 connections per relay.

Clients should only extend between two relays that support the Relay=3 protocol version. So we shouldn't need to backport this change.

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