Implement _simple_delete_many_txn, use it to delete devices

(But this doesn't implement the same for deleting access tokens or e2e keys.

Also respond to code review.

synapse/handlers/device.py

@@ -169,6 +169,40 @@ class DeviceHandler(BaseHandler):
 
         yield self.notify_device_update(user_id, [device_id])
 
+    @defer.inlineCallbacks
+    def delete_devices(self, user_id, device_ids):
+        """ Delete several devices
+
+        Args:
+            user_id (str):
+            device_ids (str): The list of device IDs to delete
+
+        Returns:
+            defer.Deferred:
+        """
+
+        try:
+            yield self.store.delete_devices(user_id, device_ids)
+        except errors.StoreError, e:
+            if e.code == 404:
+                # no match
+                pass
+            else:
+                raise
+
+        # Delete access tokens and e2e keys for each device. Not optimised as it is not
+        # considered as part of a critical path.
+        for device_id in device_ids:
+            yield self.store.user_delete_access_tokens(
+                user_id, device_id=device_id,
+                delete_refresh_tokens=True,
+            )
+            yield self.store.delete_e2e_keys_by_device(
+                user_id=user_id, device_id=device_id
+            )
+
+        yield self.notify_device_update(user_id, device_ids)
+
     @defer.inlineCallbacks
     def update_device(self, user_id, device_id, content):
         """ Update the given device