[Draft] Improve (dis-)connect handling between peers

[kaimast]

Fixes #3893.

The main purpose of this PR is to propagate errors during handshakes up, so the error is logged at the appropriate location. It also enables filtering out benign errors such as "Already connecting".

Additionally, the PR adds a four new disconnect reasons "already connecting", "already connected", "connecting to myself", "no untrusted external peers allowed". This avoids those confusing peer disconnected before sending "Message::ChallengeResponse" messages.

[ljedrz]

Some of the goals of this PR are achieved with #3900 (which is also a bugfix). In addition, I can see that some of the changes here are conflicting with the aforementioned PR, which deduplicates some of the related code.

That being said, the idea to introduce a new, concrete error type is solid, and so is the Result change in the Handshake protocol. Since I've been tinkering with peering quite a lot lately, my recommendation would be as follows:

1. close this PR for now, as the code it is applied to can still be compacted, making the change surface smaller
2. simplify/deduplicate the network-related code further (we still have plenty of duplicate functionalities between the Gateway and the Router)
3. update and extend the connectivity/peering tests to make sure they cover everything we need (it's been a long time since their introduction)
4. reconsider these changes and apply them in small chunks, without any unrelated adjustments

[vicsn]

Agreed with all of the above except:

simplify/deduplicate the network-related code further (we still have plenty of duplicate functionalities between the Gateway and the Router)

Please no refactors unless agreed upon and resolving an existing issue.

[ljedrz]

Please no refactors unless agreed upon and resolving an existing issue.

Fair enough; I was mostly thinking about the prospect of a new node type (BootstrapClient), and some of the existing issues (like #3888), which are likely to naturally lead to such a result.