Document the ACTIX_THREADPOOL env variable for use with mysql databases and change example config to set it to a large number

[fzzzy]

We're now using the actix threadpool instead of tokio-threadpool in the actix-web 2 branch. Thus, there is a different environment variable, ACTIX_THREADPOOL, we need to set to set the size of the threadpool. It defaults to num_cores * 5. We should probably set it to 128 or some other large number.

[fzzzy]

We don't actually have to do this anymore, because we don't use the actix threadpool in the spanner client any more 🎉

[pjenvey]

We may or may not need to support this for mysql, reopening for a reminder to reevaluate this later.