Detect shared secret key usage

[flub]

Sometimes users create multiple endpoints with the same secret key due to misunderstandings. To help them catch mistakes we might be able to help recognise this situation:

The connection to the relay server will exhibit a particular disconnection/reconnection pattern. It might be possible to recognise this pattern and log a warning or info message with a hint of what might be going on.

Additionally if the same secret key is used in another endpoint in the same process it could be possible to error when creating the 2nd endpoint.

[b5]

I hit this all the time, and we've had numerous users run into it as well. The current status quo is users in both endpoints get spammed with:

iroh::magicsock::transports::relay::actor: Lost connection to relay server
iroh::magicsock::transports::relay::actor: Lost connection to relay server
iroh::magicsock::transports::relay::actor: Lost connection to relay server