Issue with handling signals in containers

[jprendes]

We currently use tokio for the communication with containerd in the shim.
Many of the shims built on top of runwasi also use tokio, for example the wasmtime shim.
With our current implementation tokio::signal is broken for wasm containers created by runwasi shims.

The reason for this is a bit involved.
With our current implementation, a container inherits the global state of the shim when the container is created.
Tokio's signal handling depends on some global state, and in particular uses the self pipe trick.
This means that the pipe used for handling signals will be shared by the shim and all the wasm containers spawned by it.
We currently don't do any special signal handling inside of the shim, so this doesn't affect us where there's one container per shim.
The problem appears when one shim is managing more than one container.