Improve Dialing

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## Design notes

We have a couple of warts in our dialing system that may cause issues with the new DHT, especially while testing.
- [ ] We should fix the simultaneous dial issue and just eat the extra connections. The fact that two nodes can't dial each other at the same time is making testing painful.
- [ ] Corollary: the connection manager should trim duplicate unused connections.
- [ ] The dialer should really prefer faster transports like QUIC.
- [ ] Connections should backoff based on addresses instead of peerIDs.

## Testing mechanics

Unit tests.

## Success Criteria
- When reuseport is disabled, two peers dialing each other at the same time should succeed.
- Multiple connections should eventually be trimmed.
- If most of the network supports both QUIC and TCP, most connections should be QUIC.

Start by examining the dialing and connection-manager entry points and the existing unit tests; the issue does not name specific files. Break the work into simultaneous dialing, duplicate-connection trimming, transport preference, and address-based backoff. Done means the listed success criteria are covered by unit tests, including concurrent dialing with reuseport disabled and QUIC preference.

Written by the indexing model from the issue text.