Consider dropping connections when the threshold is full

[Stebalien]

That is, consider dropping connections from the underlying TCP socket. Maybe there's an option we can set on the TCP socket to let the kernel do this?

[Stebalien]

https://github.com/libp2p/go-libp2p-transport-upgrader/blob/5ddf5deb2271e43bf3c4ffa24841e13c69d4581f/listener.go#L116-L136

[Stebalien]

This doesn't look like the issue. Instead, it looks like a sleep in the error catcher.

[raulk]

We have a lot of piecemeal mechanisms for backpressure scattered across the codebase. At one point we'll need to consider a reusable backpressure gadget, maybe inspired by SEDA.