protocols/identify: Allow at most one inbound identify push stream

[mxinden]

Description

An identify push contains the whole identify information of a remote
peer. Upgrading multiple inbound identify push streams is useless.
Instead older streams are dropped in favor of newer streams.

Links to any relevant issues

Open Questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[thomaseizinger]

Fine with me.

Is this going towards the "remove the Upgrade traits" idea that you mentioned at some point?
Perhaps we should also build some infrastructure where protocols can define that concurrency factor of their substreams.

[mxinden]

Is this going towards the "remove the Upgrade traits" idea that you mentioned at some point?

The lazy upgrade used in this pull request, yes. Though even if we don't go all the way, I think this is a step forward.

I will write up a proposal at some point.