Remove tokio dependency from tendermint-machine

Indirects it via a minimal wrapper which can be trivially patched.

common/patchable-async-sleep/src/lib.rs

@@ -0,0 +1,10 @@
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![doc = include_str!("../README.md")]
+#![deny(missing_docs)]
+
+use core::time::Duration;
+
+/// Sleep for the specified duration.
+pub fn sleep(duration: Duration) -> impl core::future::Future<Output = ()> {
+  tokio::time::sleep(duration)
+}