async'ify the groups code

src/users.js

@@ -381,16 +381,16 @@ function get(userId, callback) {
     assert.strictEqual(typeof userId, 'string');
     assert.strictEqual(typeof callback, 'function');
 
-    userdb.get(userId, function (error, result) {
+    userdb.get(userId, async function (error, result) {
         if (error) return callback(error);
 
-        groups.getMembership(userId, function (error, groupIds) {
-            if (error) return callback(error);
+        let groupIds;
+        [error, groupIds] = await safe(groups.getMembership(userId));
+        if (error) return callback(error);
 
-            result.groupIds = groupIds;
+        result.groupIds = groupIds;
 
-            return callback(null, result);
-        });
+        return callback(null, result);
     });
 }
 
@@ -471,16 +471,11 @@ function update(user, data, auditSource, callback) {
     });
 }
 
-function setMembership(user, groupIds, callback) {
+async function setMembership(user, groupIds) {
     assert.strictEqual(typeof user, 'object');
     assert(Array.isArray(groupIds));
-    assert.strictEqual(typeof callback, 'function');
 
-    groups.setMembership(user.id, groupIds, function (error) {
-        if (error) return callback(error);
-
-        callback(null);
-    });
+    await groups.setMembership(user.id, groupIds);
 }
 
 function getAdmins(callback) {