fix: prevent duplicate connections to multiaddr

[tabcat]

Title

fix: prevent duplicate connections to multiaddr

Description

After opening a new connection, check if there is an existing connection
to the same peer. If the existing connection is not limited or the new
connection is also limit, return the existing connection.

This will prevent multiple connections to the same peer in cases where
the multiaddr being dialed does not contain a peer id.

Notes & open questions

Closes #2714

There is a small change from what was requested in the issue. An existing limited connection will be returned if the new connection is also limited.

Should conn.close() be awaited before returning the existing connection?

Where should a test for this be added?

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

[tabcat]

I've added two tests to check that the existing or new connection is returned appropriately.

[achingbrain]

An existing limited connection will be returned if the new connection is also limited.

In practice this shouldn't happen since the only limited connections we have are circuit relay connections, and we can't dial one without the target peer being in the multiaddr (as the relay needs to know which onward peer we are trying to dial), so the connection should already have been de-duplicated based on the target peer id in the multiaddr and a second relayed connection not opened.