In steady-state, requests to FCM take about a second; however, in
cases where the remote FCM server is unstable, the request has been
observed to block for more than a minute.
As noted in the previous commit, pushes must complete within 30s;
fail fast, and let the retries and exponential backoff handle errors.
The worst-case total time taken with timeouts and errors for an FCM
notification is 19.5s. Unfortunately, `aioapns` does not appear to
have any timeouts, and thus this commit cannot guarantee a total of
fewer than 30s.