makeBackoffMachine [nfc]: Use sleep() helper rather than re-implement.

Move `sleep` and `delay` to top of async.js and use `sleep` in
makeBackoffMachine, to be concise, instead of re-implementing the
same thing.

src/utils/async.js

@@ -1,6 +1,14 @@
 /* @flow strict-local */
 import { isClientError } from '../api/apiErrors';
 
+/** Like setTimeout(..., 0), but returns a Promise of the result. */
+export function delay<T>(callback: () => T): Promise<T> {
+  return new Promise(resolve => resolve()).then(callback);
+}
+
+export const sleep = (ms: number = 0): Promise<void> =>
+  new Promise(resolve => setTimeout(resolve, ms));
+
 /**
  * Makes a machine that can sleep for a timeout that, until a ceiling is reached,
  * grows exponentially in duration with the number of sleeps completed, with a
@@ -47,21 +55,13 @@ export const makeBackoffMachine = () => {
         firstDuration * base ** numSleepsCompleted,
       );
 
-      await new Promise(resolve => setTimeout(resolve, duration));
+      await sleep(duration);
 
       numSleepsCompleted++;
     },
   };
 };
 
-/** Like setTimeout(..., 0), but returns a Promise of the result. */
-export function delay<T>(callback: () => T): Promise<T> {
-  return new Promise(resolve => resolve()).then(callback);
-}
-
-export const sleep = (ms: number = 0): Promise<void> =>
-  new Promise(resolve => setTimeout(resolve, ms));
-
 /**
  * Calls an async function and if unsuccessful retries the call.
  *