Rewrite olmlib.ensureOlmSessionsForDevices as async

This is non-functional. It just looks a lot prettier.

src/crypto/olmlib.js

@@ -121,7 +121,7 @@ module.exports.encryptMessageForDevice = async function(
  *    an Object mapping from userId to deviceId to
  *    {@link module:crypto~OlmSessionResult}
  */
-module.exports.ensureOlmSessionsForDevices = function(
+module.exports.ensureOlmSessionsForDevices = async function(
     olmDevice, baseApis, devicesByUser,
 ) {
     const devicesWithoutSession = [
@@ -151,7 +151,7 @@ module.exports.ensureOlmSessionsForDevices = function(
     }
 
     if (devicesWithoutSession.length === 0) {
-        return Promise.resolve(result);
+        return result;
     }
 
     // TODO: this has a race condition - if we try to send another message
@@ -161,9 +161,10 @@ module.exports.ensureOlmSessionsForDevices = function(
     // That should eventually resolve itself, but it's poor form.
 
     const oneTimeKeyAlgorithm = "signed_curve25519";
-    return baseApis.claimOneTimeKeys(
+    const res = await baseApis.claimOneTimeKeys(
         devicesWithoutSession, oneTimeKeyAlgorithm,
-    ).then(function(res) {
+    );
+
     const otk_res = res.one_time_keys || {};
     const promises = [];
     for (const userId in devicesByUser) {
@@ -205,8 +206,9 @@ module.exports.ensureOlmSessionsForDevices = function(
             );
         }
     }
-        return Promise.all(promises).return(result);
-    });
+
+    await Promise.all(promises);
+    return result;
 };
 
 async function _verifyKeyAndStartSession(olmDevice, oneTimeKey, userId, deviceInfo) {