ISM: Task tracking discards

The task tracking for async tasks is currently disfunctional. It appears to get confused when too many tasks are scheduled. Logs have also indicated that it may need special handling for some Sync messages like Echo.

This has also been materialized in recorded metrics (before the agent, from the gateway) being skewed. This makes the Grafana Graphs at the moment effectively useless.

---

The relevant code is in gateway/proxy.go. @blohmar would you like to pick this up?

You previously mentioned being interested in refactoring the proxy logic and this appears to be a good time for it. With !7 (merged) being bound to the executor contract, this should also be independent of any executor changes.