Async (remote) cache writes

[stuhood]

Currently the remote Cache writes synchronously to the cache when it receives a successful result. This write should be async.

Making the write async is trivial: adding a tokio::spawn that the runner does not wait for... i.e.:

let _cache_write = tokio::spawn(async move { /* ... write to the cache */ });

Testing the change, is slightly more challenging. Two potential options:

1. Adding a delay to the StubActionCache for cache writes, and then demonstrating that CommandRunner::run takes less than that delay (and that the StubActionCache does still receive the write).
2. Extracting a trait for the "Cache" implementation itself, and using a mock cache implementation in the test that you use to validate that the run method completes before the cache write.

The latter implementation is more general, and has some other benefits: with an extracted Cache implementation, we could use the CommandRunner implementation for both local and remote running, with both speculation and async writes.