Score a path through TURN as a relay pair

[SashaMIT]

Summary

A host candidate paired with a remote relay was scored as the average of host and relay. That average is 55, so the TURN path outranked a server-reflexive pair at 50 even when the direct pair had the lower round trip time. Automatic renomination kept selecting the relay.

A path that uses a relay candidate now scores as relay. A succeeded path that does not use TURN outranks every relayed pair, and renomination will leave the relay for that path. Two candidates that are both direct still rank by their type average and round trip time. A relay is not selected in place of a path that is already direct.

TestDirectPairBeatsPairWithRelayOnOneSide failed before this change: the direct score 53.98 was not greater than the relayed score 57.52, and the best pair was the relay. With the change, the direct pair scores higher and is selected. TestCandidatePairQualityTypeScores and the existing renomination cases passed. golangci-lint run --new reported 0 issues.

The controlling agent already has both pairs in the succeeded state. The score was keeping media on TURN after a direct path was up.

Fixes #1033

Test plan

• Host paired with a remote relay loses to a server-reflexive pair with a lower round trip time
• Existing type-score matrix and relay-to-host renomination cases