Use atomic.Bool to store whether a rungroup actor has already been interrupted

[RebeccaMahany]

Many of our rungroup actors contain interrupt chan struct{} and interrupted bool fields within their structs. We use the interrupted bool to prevent accidentally running shutdown routines twice; rungroup actors should be able to have their shutdown functions called multiple times.

However, interrupted bool can cause data races in tests. We should update to use atomic.Bool in all rungroup actors that have an interrupt and interrupted field.

We can use James's work in #1977 as an example.