fix: avoid double conns, better state tracking

[Frando]

Description

On main the test sync_full_basic is flakey. This PR (hopefully) fixes it.

The reason was: We have the situation that two peers initiate connections to each other at (roughly) the same time. In #1491 this was sometimes prevented, but not reliably.

This PR fixes it by:

• When connecting, we set a SyncState::Dialing for the (namespace, peer)
• When accepting, if our own state is Dialing for the incoming request for (namespace, peer) we compare our peer id with that of the incoming request, and abort if ours is higher (doesn't matter which way, we care about a predictable outcome only
• Through this, only one of the two simoultanoues connections will survive
• Also added a Abort frame to the wire protocol to transfer to inform the dialer about the reason of the declined request, which is either "we do double sync, and will take the other conn" (AlreadySyncing) or "this replica is not here" (NotAvailable)

This PR also:

• Further improves logging
• Improves errors

Notes & open questions

Change checklist

• Self-review.
• Documentation updates if relevant.
• Tests if relevant.

[divagant-martian]

the only things that jump to me is doing explicit imports of std::result::Result if you are using anyhow::Result, setting the error will overwrite the generic so that's not necessary (if that's the reason)