fix prioritization of stream muxers

[marten-seemann]

Looks like we've been prioritizing mplex over yamux since at least June 2020. This is really, really bad.

[Stebalien]

Well, this is my new "most embarrassing mistake". I can confirm, and have no idea how we failed to notice. I guess most of the pprof profiles we've been looking at are from the gateways.

We should get this fix in, write a test.

[guseggert]

Is there a way to add a test for this?

[Stebalien]

Is there a way to add a test for this?

At a minimum, we can write a unit test for the prioritization logic. But we probably need to export some additional stats in libp2p all the way through the swarm for better tests.

[marten-seemann]

I added a unit test for the prioritization logic.

[BigLep]

This should get called out in the release notes, basically that:

go-ipfs-new → go-ipfs-old|new will now use yamux instead of mplex.